The Queen of Ireland couldn't have timed its Northern Ireland release better. The documentary about marriage equality in the Republic of Ireland arrives in cinemas the same week the DUP scuppered a majority Assembly vote to allow same-sex weddings in the north.

After watching this deeply affecting film, the anger, sadness and frustration felt by many at the party's underhand use of a petition of concern will be intensified.

Director Conor Horgan's beautifully shot and edited movie follows Panti Bliss, the drag queen alter-ego of Co Mayo-born performer and activist Rory O'Neill, who somewhat inadvertently became the LGBT movement's figurehead in the run-up to May's marriage rights referendum.

In her towering heels and extravagant, blonde wig, she is an imposing presence, yet O'Neill's larger-than-life character is as persuasive as she is visually arresting.

In his own words, Panti is a "giant cartoon woman", but she is also an eloquent and incisive commentator, who counts the likes of Stephen Fry and Madonna among her legion of fans, and in 2014, received an Irish Person of the Year Award.

Her creator's life has certainly been an eventful one. The Queen of Ireland takes us from O'Neill's childhood in the small town of Ballinrobe, where he was, as he puts it, "the local gay", through the perhaps inevitable art college years, to the development of his stage persona during hedonistic adventures in London and Tokyo.

Eventually, O'Neill comes home to a relatively more progressive Ireland and embarks on a campaigning trail that ultimately leads to the Republic becoming the first country to approve same-sex marriage through a public vote.

The Queen of Ireland isn't just powerful because of the emotive subject matter; it has a rich dramatic arc, too.

There is tragedy when O'Neill suffers a serious health setback, and when he invokes costly legal proceedings with contentious remarks made on RTE's Saturday Night Show, a row that is dubbed "Pantigate".

But there is triumph when he returns to Ballinrobe to perform to a sold-out crowd in a marquee in a car park near his family home, and later, when the 'Yes' result is returned in the referendum.

As a stand-up, Panti is smart and hilarious, albeit one you might not take your mother to see (and indeed, O'Neill tones down the swearing and explicit sexual references for the homecoming gig, attended by his elderly parents).

Panti's abrasive one-liners earn The Queen of Ireland its 15 certificate, but behind the facade, O'Neill reveals a complex personality. He is as humble and kind as his self-described "court jester" drag act is outrageous.

It may be Horgan's film, but it's O'Neill and Panti's show, and as narrator, the cross-dressing star steers the narrative to its startling denouement - Ireland's legalising of gay marriage. To see same-sex partners celebrating in streets where 22 years previously homosexuality had been punishable by prison delivers an emotional punch on a par with any feel-good flick.

If you'd pitched this tale to a Hollywood producer in the early Nineties, you might well have been laughed out of the room.

The realities of being a gay man or woman in Ireland in the Seventies and Eighties are well covered through extensive interviews and newsreel footage, and it's heartening to see how far Irish society has come, though for audiences in the north, it will be dispiriting to be reminded how far we are lagging behind.

The Queen of Ireland deserves to be seen by everyone, be they gay, straight, male, female, young or old. In fact, this important piece of work should be shown in schools - and maybe even in the Northern Ireland Assembly.

Four stars

Belfast Telegraph