If anyone needs proof that Irish actress Katie McGrath’s star is on the rise then they only have to look at the company the Wicklow beauty has kept in recent times.

Drinking pints with Sin City star Clive Owen, partying with Madonna at her 52nd birthday party and being chatted up by Roger Moore — “a roguishly charming man, but in a lovely way” — all go some way towards proving that this 27-year-old isn’t doing too badly.

But for Ashford girl Katie, the one achievement that truly makes her smile is appearing on postage stamps in the UK. Yes, postage stamps. Just like the Queen. “It’s so weird that my face is on a stamp,” she laughs self-consciously, when LIFE magazine meets the stunningly gorgeous star in a plush hotel in central London. “It’s even weirder that you can get matching envelopes with me on them as well,” she adds. And while that news might come as a bit of a surprise to us over here, where, relatively, Katie has flown under the radar, in the UK and beyond, the Trinity graduate is seen as one of the hottest talents to have emerged from this island in recent years.

So hot, in fact, that Madonna dropped another actress to cast Katie in her directorial debut WE, which tells the story of King Edward’s controversial lover, American divorcee Wallis Simpson. And so in demand is Katie that she landed the lead role opposite former James Bond Sir Roger Moore and Scottish heart-throb Sam Heughan in this Christmas’s rags-to-riches sure-fire hit, A Princess for Castlebury. And all that’s not even mentioning the fact that, for the last four years, seven million fans have every Saturday tuned in to BBC1 costume drama Merlin, to watch Katie play manipulative minx Morgana in the medieval series. The drama, which is loosely based on the legend of King Arthur and Camelot — albeit with a lot more talking dragons — is so popular that it was recently sold to 52 broadcasters in 180 countries, including China and the United States.

There is even a dedicated, if slightly intense, online Merlin community that cannot get enough of all things Morgana. Hence the stamps. But how did a self-professed tomboy, who grew up with posters of grunge rockers Green Day on her walls, achieve such global domination, especially when she only began acting at the relatively late age of 23? “I wasn’t born an actress but I was certainly born dramatic!” Katie giggles, as we sit down together for a breakfast of orange juice, tea and a selection of pastries. Tall, dark and slim, I’m immediately struck by how her animated manner is at odds with her serene, ethereal beauty. She jokes constantly, gesticulates wildly and finishes most sentences with a searching, “Do you know what I mean?” before collapsing into giggles. But there is something else that I can’t put my finger on. It’s only as I watch her speak that I realise she bears an uncanny resemblance to Atonement star Keira Knightley. Has she heard that before? “Oh yeah, people do say that all the time.

But to be compared to one of the most beautiful women in the world? I’m like, ‘More of that, please!’” she says. Yet while Keira and Katie could indeed be sisters in appearance, Keira was born into an acting family, whereas Katie had a much more unexpected route to stardom. Born the youngest of three children to dad Paul, who works in computers, and mum Mary, who works for Irish designer Lainey Keogh, Katie is the only girl in the family and “daddy’s little princess”, she quips. She is very close to her older brothers, post-production producer Rory, 30, and online media manager Sean, 32. In fact, the three siblings shared an apartment for a time while she studied history at Trinity College. Rory was even in her year, although he studied philosophy, while eldest brother Sean studied psychology at UCD.

Yet despite living with her adored brothers and having a passion for her chosen area of Russian history — she switches impressively between discussing the best way to film a sex scene, to the emancipation of the serfs in 19th-Century Russia — Katie admits she wasn’t happy at university. “To be honest, I wasn’t in the right frame of mind to be in college,” she says, absent-mindedly twirling her hair, an endearing habit that she later insists is one of her most annoying traits. “I only went because I didn’t know what else to do. While I was there, I was always looking for ways to get my life going.” These ways included trying out a variety of careers for size, including cocktail barmaid, make-up artist and working in a vintage-clothes shop. Katie even tried her hand as a tattoo artist, which is when she got the word “Purity” inked in large black letters across her lower back, decorated with two black stars on either side.

I’m intrigued as to why she chose the word “Purity”. Does it represent a mantra for life? “Oh, God, no,” she exclaims. “Hell, no! Let’s clear that one up right now. It is not a comment on me as a person and it’s not something I live by. Life would be very dull if it was, wouldn’t it?” she asks. It would indeed, but why get it then? “Katie means ‘pure’ in Greek,” she explains, as she lifts her tight brown top to show me the dark cursive script across the curve of her spine. “I researched my tattoo and I got it for the right reasons and after 10 years, I still absolutely love it.” I wonder if one of the reasons she’s so fond of her tattoo is that it would later spare her blushes — well, sort of — in a sex scene with actor Jonathan Rhys Meyers on her first acting job in The Tudors. But we’ll come back to that.

Meanwhile, after graduating, Katie thought she had found her calling when she was given work experience on Image magazine. “I had this vague notion that one day I might be editor of Vogue China,” she chuckles. “It was a bizarre ambition, as I didn’t speak a word of Chinese. There were flaws in my plan, admittedly.” But nine months later, restlessness struck and Katie left the magazine. “It just wasn’t for me,” she explains. “It was like, ‘I’m not going to be a tattoo artist, now I’m not going to be a fashion journalist either.’ I guess it was another one crossed off the list.”

Stuck without a job, her mum’s friend found her a position in the wardrobe department of the historical drama The Tudors. “I knew nothing about working with clothes,” Katie explodes with laughter as she remembers the panic on her first day. “I was asked to take up a hem and I was like, ‘Oh God, how do I do that?’ I mean, I knew the theory behind stitching but actually doing it was completely different.”

But Katie’s world was about to change. At the end of the first series, she was asked to stand in for some publicity imagery. “There was a shortage of actresses on set that day so they asked me to dress up as a busty wench for some photos. The pictures must have gone well, because afterwards some of the producers said to me I should try out for an actual role on the series. So I wrote this really cringe-worthy letter to all the agents in Ireland telling them why I should be an actress and why they should take me on. Thank God one of them replied.” That agent was Josh Muldoon at TN Enterprises (although Katie is now with The Rights House in London). He clearly spotted her natural charisma and soon she was back on the set of The Tudors — but this time as sexy commoner Bess.

“The Tudors being The Tudors, pretty soon my character was having sex with Jonathan Rhys Meyers,” she grins. “I was lucky, though, because the make-up department spent two hours covering up my tattoo with body make-up, but as soon as I put on my corset, it rubbed off immediately. As there were no tattoos in Tudor times, the director couldn’t do all the shots they wanted in the bedroom. So I was like, ‘Dammit! I have to keep my clothes on!’” she feigns disappointment while laughing, “I totally dodged a bullet.” Nevertheless Katie still had to grin and bear it in a fairly explicit scene as sexy Bess purrs at King Henry, “Are you really the King of England?” “There is always that awkward moment at dinner when someone’s parent says they’ve seen me in The Tudors,” she confides. “It’s a bit like, ‘Oh, God, how do I continue the conversation from here?’”

The onscreen shenanigans in the series bonded the cast closely and soon there was speculation that life was imitating art, after Katie and Jonathan Rhys Meyers were pictured with each other, protesting at the Hill of Tara and partying at Electric Picnic. “We were never together,” Katie insists. “There were stories in the papers at the time but we’re just really good mates. Is Jonny my type? I guess so. He’s really beautiful and smart. And I like smart.” The pair remain close friends, and Katie is clearly upset when I mention Jonathan’s recent struggles. “It’s been difficult, because I’m filming and away and he’s filming and away, and friendships like that are always hard,” she says. “It is a really difficult time for him and, you know, you just want your friends to be OK and I just hope that he is.

I’ll text him or call him just to say, ‘Listen, I’m here.’ He’s a really sensitive guy and I think that’s what makes him such a great actor. He uses the pain in his work.” And it was her memorable scenes with Jonathan that proved a springboard for her new acting career as, a few short months later, she won the role of blueblood witch Morgana on BBC1’s newly commissioned Arthurian drama Merlin. “It all happened so quickly,” she reflects. “I got the job on Monday, moved to London on Friday and started filming the next Tuesday. When I moved over I didn’t know a soul in London. For the first six months, if anyone asked me to meet for coffee, I was like ‘Great! When? Now? Now? Now?’” Looking back, she is clearly able to laugh, but it can’t have been easy being a lonely newcomer in a big city. Thankfully, it seems that her biggest concern these days is whether or not to splash out on a vintage 1965 Ford Mustang. “I’m a total petrolhead,” she confesses. “Deciding whether or not I should buy that car was honestly the best half hour of my week.”

She didn’t buy the car, but with friends like Bedlam actress Charlotte Salt, Silent Witness star Emilia Fox and Ifta-nominated actress Jenn Murray down the road in leafy Fulham, who needs to drive? And when her schedule for Merlin allows, Katie likes to relax in London’s exclusive members’ clubs such as Century and Soho House, where she bumps into enigmatic Sherlock star Benedict Cumberbatch and parties with the likes of designer Matthew Williamson and actor Clive Owen (“I embarrassed myself by asking him if he knew about Merlin, forgetting of course that he played Arthur in King Arthur!”) And not forgetting, of course, hanging out with the queen of pop. “Madonna is totally not what you expect,” Katie reveals. “When I went to audition for the part of Lady Thelma Furness last year, she actually lay down on the floor of her dressing room with me to recreate a scene where my character is in bed with King Edward. I was lying on the ground with Madonna and this supermodel, who was playing my twin sister, pretending we were all in a bed together. It was a completely surreal moment, to say the least.”

And Madonna took to talented Katie so much that when the global superstar turned 52 last August, she invited her new friend to her birthday party at Shoreditch House. This left Katie with an enviable dilemma: what do you buy Madonna as a birthday present? “I didn’t know what to get her,” Katie confesses. “Eventually I decided on a 50th anniversary edition of To Kill A Mockingbird. I thought that even if she has read it, it’s such a good book and everybody loves the story.”

Was the Material Girl hard to work for? “People perceive Madonna very differently to what she is actually like,” Katie insists. “She’s much more relaxed than you’d think. She would talk to everyone on set, she knew everyone’s name. She loved that I was Irish. She’s really down-to-earth. Yes, she got stressed, but no more than any other director I’ve worked for. It’s a stressful job.” After filming wrapped on that once-in-a-lifetime experience, Katie then flew to Romania to star opposite another legend, Roger Moore. In the film A Princess for Castlebury, which will be released this December, Sir Roger plays the elderly father of Katie’s love interest, though this didn’t stop the infamous charmer from flirting with beautiful Katie. “He is still exactly what you would think, a sort of slightly roguishly charming older man, but in a really lovely way,” Katie laughs. “It was amazing. I was like, ‘Roger Moore’s flirting with me!’” she mock squeals. “There’s nothing behind it, of course, because he’s married, but, still, Roger Moore is flirting with me!”

But for a girl who caught the eye of The Spy Who Loved Me star, incredibly Katie is single. And even more incredibly, she comes over all self-conscious and shy when I mention dating. “I am really terrible when it comes to guys,” she confides wistfully. “Inside I just see myself as this overweight tomboy with funny coloured hair and bad skin.” Ah now, come on. Overweight with bad skin? I don’t think so. “No, seriously,” she insists. “I was really big until I left college and I started to lose all the weight. I don’t know how big I was exactly. Let’s just say when I stopped fitting into a size 14, I stopped counting. Because of that I always saw myself as ‘the friend’ rather than the one that guys wanted to talk to. I was very awkward and guys were always just my mates.

Inwardly, I haven’t changed at all, so when I talk to guys now I treat them the same way. And then there’s this moment when I realise, ‘Oh, this isn’t what I think it is, oops, how do I get out of this?’” Would she like a relationship? “One day I’d love a house in the country, with some chickens, dogs and kids,” she says thoughtfully. “And, you know, I’m approaching that age now where all my friends are getting married and I’m like, ‘When did that happen?’ And I am terribly broody, horrifically, horribly broody,” she says with a laugh. But for now, it’s work, work, work for Katie, with the odd bit of socialising squeezed in. The day after we meet she is heading to music festival The Big Chill in Herefordshire where her favourite band, Fight Like Apes, are playing. “They’re first up on the bill and I’m so excited,” she gushes. “I have this unbelievable girl crush on the singer Mary-Kate Geraghty. I got really drunk at one of their gigs in Belfast a few years ago and cornered her, telling her how amazing she was. The band kept smiling at me all night and I couldn’t really work out why, but then I read in an interview afterwards that they thought I was Keira Knightley. What a disappointment for them!”

A disappointment? After spending a morning with the engaging and beautiful Katie McGrath, I very much doubt it.

Photography by David Cantwell Assisted by Naomi Dunleavy

Styling by Liadan Hynes Assisted by Jessica Gaffney and Jessica Finn

Make-up by Paula Callan, Brown Sugar, 50 Sth William St, D2, tel: (01) 616-9967 Hair by Paul Davey, Davey Davey, 23 Drury St, D2, tel: (01) 611-1400

Shot at Number 10, Dublin’s most exclusive private venue. Available for intimate weddings and private parties, see www.no10dublin.com