I was on the train home a while ago, enjoying the rare peace of an almost-empty carriage. A younger man seated opposite me was hunched over his laptop, headphones clamped to his ears. I noticed that whenever someone walked through the carriage he would glance up and angle the laptop fractionally closer to his chest.

As the light outside faded, and the reflection cast by his screen on to the dusty windows brightened, I understood why: projected on to the glass were blurry but unmistakable images of naked, writhing bodies.

I was surprised he felt entitled to watch pornography in public. I was slightly uncomfortable to be alone in a carriage with someone who was presumably in a state of sexual excitement. But at least he was trying to be discreet about it.

The man was openly scrolling through graphic, pornographic photographs of women in sexual positions and women masturbating

I forgot about the incident until a colleague had a similar experience on the Luas home late at night. She got into a quiet part of the tram and made for the first available seat. A man in his 60s had to move his briefcase to let her sit down. As they were sitting in adjacent seats her eyes inevitably flickered towards the images he was openly scrolling through on his phone.

They were “graphic, pornographic photographs” of women in sexual positions and women masturbating, she says. “He made no attempt to hide it,” and although he was “deep into it” he was certainly aware of her presence.

She didn’t move seats or challenge him, partly because she was more professionally intrigued than distressed by it and partly because she was travelling only a couple of stops. She also wasn’t sure whether he could be challenged. He was watching the images on his own phone, and although he wasn’t hiding them he wasn’t forcing anybody to look.

Is it ever okay to watch porn on public transport? And, if somebody decides that’s how they want to pass their commute, is there anything, other than embarrassment or a sense of decency, to suggest they shouldn’t?

There is, unsurprisingly perhaps, no legislation or bylaw directly governing “looking at porn on public transport”.

Section five of the Criminal Justice (Public Order) Act of 1994 bans “offensive conduct” in a public place – defined as “any unreasonable behaviour which, having regard to all the circumstances, is likely to cause serious offence or serious annoyance to any person who is, or might reasonably be expected to be, aware of such behaviour”.

But, as a legal expert points out, the ban applies only between midnight and 7am unless a garda has asked the person to stop. So you’d have to find a garda to ask the person beside you to stop looking at porn on their phone, and they’d have to refuse the request for their behaviour to be an offence.

On a flight in the US last year cabin crew had to ask a man sitting beside a 16-year-old girl to stop watching hard-core pornography on his phone

There’s also section 45 (3) of the Criminal Law (Sexual Offences) Act 2017, according to which a “person who intentionally engages in offensive conduct of a sexual nature” – any behaviour “likely to cause fear, distress or alarm” to anyone aware of it – “is guilty of an offence”. But it’s far from clear whether looking at pornography on a personal device constitutes “conduct”, how what people look at on their phones could be policed, or even whether we’d want to try to police it, given the obvious privacy concerns.

Transdev, the Luas operator, says its customer-care teams have never received any complaints about people watching pornography, although they frequently get complaints about everything from people hogging seats with their bags to people listening to RTÉ News too loudly on their headphones.

Whether people should be watching porn on public transport is, then, down to respect for other passengers. “It’s really a matter of etiquette,” a Transdev spokesperson says. “What is appropriate and what is not?”

It’s also, frequently, about power and entitlement.

On a flight in the US last year cabin crew had to ask a man sitting beside a 16-year-old girl to stop watching hard-core pornography on his phone. According to a report of the incident published by one of the crew involved on the website Flyertalk, the man agreed, but continued watching anyway.

“It wasn’t so much the content of the film but the intent of the man in the act of watching it plainly in view of a captive, young female audience,” wrote the flight attendant, Amanda Pleva, getting straight to what is arguably the nub of the issue. It’s not so much about what the person is watching as about what they’re saying by watching it in public, despite knowing the effect it might have on those around them.

The man had to be asked twice to stop. “We confronted him and threatened him with arrest if it continued, and he complied. The girl was moved to sit near her mother, where she sat nervously and cried,” Pleava wrote.

Was the young man on my train, who seemed convinced no one else could see him, doing anything wrong? I didn’t think so then, and I still don’t now

Ultimately, the issue comes down to balancing privacy and personal freedom on the one hand with common sense, good manners and respect on the other. What you choose to watch or do on those devices we increasingly regard as extensions of our private selves is usually your own business, as long as you do it discreetly. (Take my advice and don’t google “porn on public transport” while on public transport.)

But privacy is not an absolute right. The moment you make your porn watching a public act, your privacy is no longer the only thing at stake, and other rights come into play.

Was the young man on my train, who seemed convinced no one else could see him, doing anything wrong? I didn’t think so then, and I still don’t now. I was the only person nearby, and all I could see were blurry images reflected in a window. Watching pornography in a public place is never polite or particularly tasteful, but in that case it wasn’t harassment.

But where is the line between private entertainment and act of harassment? Could you argue that the man on the Luas crossed it by feeling entitled to explore a private sexual interest in public? Possibly. The man on the plane? Undoubtedly. Or what about the person who scrolls through YouPorn videos using their private device during their lunch break at work, but in a place where their colleagues might be reasonably expected to see?

If in doubt there’s a simple guideline to help you navigate the etiquette of watching porn in public. Don’t. This seems like a statement of the obvious, but, judging by the conversations I’ve had on this topic, there are men out there – some of them on a Luas or a bus near you, right now – who feel entitled to watch whatever they want, whenever they want, as blatantly as they want. (There may be women watching porn in public, too, but I have yet to hear of them.)

To those men I’d say, watch all the pornography you want when you’re alone. But if you choose to view hard-core videos or graphic imagery in a public place, where you are likely to be seen by someone who could reasonably feel threatened or disturbed by it – and whether they’re entitled to feel threatened or disturbed is a call that is never yours to make – it stops being a private act and becomes an act of harassment.