When the killing's finally over, what do you write about? It's a question confronted by dramatists in any post-conflict society, but one that's particularly raw and relevant in Northern Ireland, where ‘The Troubles’ continue to emit their potent aftershocks on a virtually daily basis.

Green Shoot Productions' ‘Ulster Trilogy’ is intended to address this situation, providing “an audit of where Northern Ireland is today, 18 years after the ceasefires”. Following 2012’s Brothers in Arms by Sam Millar and Paisley & Me by Ron Hutchinson, Meeting at Menin Gate, by the company's artistic director Martin Lynch, is the trilogy's final instalment, and examines the ongoing predicament of victims and perpetrators in the Northern conflict.

Lynch's conclusions are crushingly negative. Liz is the Protestant daughter of an RUC policeman shot dead in the Troubles. Terry is an ex-Republican activist. They meet on a trip to World War I battlefields organised by ‘The Flanders Society’, a cross-community organisation in Belfast. The two swap stories, eat dinner together, and have sex in a field of cornflowers. So far, so positive, for the truth and reconciliation process the Flanders excursion is meant to propagate.

Things then turn ugly. Liz discovers that Terry's surname is the same as one of the two men suspected of shooting her father, but never convicted. Convinced of Terry's involvement, she is wracked with nausea, and in a tense climax to Act One confronts him with her horrified suspicions.

What follows after the interval is the most gratuitously unpleasant fifty minutes I've ever spent in a theatre. In a gruesomely extended sequence set in a Belgian hotel room, we witness the torture of Terry, drugged by Liz, bound with rope, then stripped of belt, trousers and underpants. Her aim? To elicit a confession from Terry, and an admission that he is a murderer pure and simple, not a politically motivated freedom fighter. Her methods? They are not subtle, and involve pliers, a staple-gun, and a set of hair straighteners. Beyond that I will spare you the gory detail.

Terry eventually confesses to the killing of Liz's father, and begs forgiveness. He is met with silence, and remains immobile, presumably dead, as Liz spins crazily round the room to a soundtrack of The Jam's ‘Town Called Malice’ and an aria from Verdi's La Traviata.

What is Lynch intending to communicate in this painfully protracted episode? If he's aiming to emphasise the depth and durability of the trauma inflicted on Liz by witnessing her father's shooting, then the point is made adequately long before the more excruciating moments in the torture sequence happen. That makes the rest of it dispensable.

As it is, neither Liz nor Terry appears to give an inch in their respective positions as Act Two progresses, and there's an oppressive sense of stasis hanging over the act dramatically. Is this intended to mirror the broader political situation in post-ceasefire Northern Ireland? Lynch himself, in a programme note, seems unsure of the extent to which his main characters may be more generally representative. He must surely be aware, however, that they will inevitably be taken that way by many in the audience.

What's certain is that Act Two is dramatically turgid, the torture scenes graphically realistic but prolonged to minimal narrative purpose. Actors James Doran and Andrea Irvine battle doggedly through them, but you sense their cumulative weariness with the process: there's only a certain quantity of nastiness you can summon, a certain range of variations you can muster on the moaning and groaning spectrum, before the emotional hurt locker is emptied.

Earlier, before the interval, both Doran and Irvine inhabit very different characters – relaxed, garrulous, flirtatious, scarred but capable of experiencing happiness in the present and hope for the future. Both respond winningly to Matt Torney's crisply conversational direction. It's a shock to see Liz totally mutating into the vicious, animalistic torturer of Act Two. Most victims, we know, are not like that. Why does Lynch make this particular one a raving psychotic? Victims present in the audience on the evening might easily have asked themselves a similar question, and been at the very least bemused at the concentrated savagery.

Doran and Irvine are colourfully supported in a succession of small cameos by the multi-tasking Marty Maguire and Maria Connolly, for whose character Cara (Liz's friend) Lynch concocts a steady stream of spectacularly offensive profanities. Niall Rea's sets are effectively minimal in Act One (two chairs, a stone arch backdrop), with a blandly well-appointed hotel room for the Act Two confrontation. You wonder, incidentally, how the liberal amounts of yelling and howling generated by both characters could possibly go unnoticed by the other residents, who are unaccountably impervious to the racket Liz and Terry are creating.

Meeting at Menin Gate opened at the MAC in Belfast not long before the US diplomat Richard Haass arrived in the city, tasked with attempting to loosen some of the Gordian knots, both psychological and political, still choking the post-conflict city. If there's a temptation, in the mind of some well-meaning local functionary, to treat Haass to a viewing of Menin Gate as part of his evidence-gathering process, it should be resisted. The play's grim, implacable vision of fatally entrenched positions and terminally damaged individuals is liable to put him on the first flight home again.

Terry Blain is an arts journalist and cultural commentator, contributing regularly to BBC Music Magazine, Opera magazine, the Belfast Telegraph, Culture Northern Ireland and BBC Radio Ulster.