There’s a strand of speculative fiction that plonks you straight into a boggling scenario, then gives you dribs and drabs of information to decode it, before laying the whole thing painstakingly out. That’s where Simon Stephens’s irritating play comes from. It’s overlaid by self-conscious quirkiness.

Two men cower in a bunker after some unspecified holocaust, testily acting out ritualised exchanges to stave off despair. If the scenario vaguely recalls Samuel Beckett’s Endgame, the characters are more like the two clowns in Waiting for Godot, only younger.

Indeed, it seems they were students when nuclear war or climate change or something else devastated the world. Both are childlike though Reece (Ian Toner) apparently has a surly teenage son, whom he vaguely hopes to see again one day, and bequeath his crappy but coveted digital watch.

He and Ashton (Ross Gaynor) flirt, bicker and dance, performing dialogue about Christmas celebrations, university interviews and a surprise pregnancy. They’re Irish-accented in Rex Ryan’s production for the Dublin company Glass Mask, but the town they live in is imagined: features include “the Chelsea Mines” and “Alaska Beach”.

They have a vase of plastic tulips, a paraffin stove to cook rice, and one LP, the greatest hits of 1930s vocal harmony group The Ink Spots, but no power for their record player. There is no sun or rain outside any more, only winds and fires.

In due course, the fragile equilibrium of their world is unbalanced by the arrival of a stranger, Presley (Fionn Ó Loingsigh), who has a gun, a portable generator and some “pork”. Andrew Clancy’s set is dominated by a huge, slatted shutter through which ominous red light seeps. Like the gun, you know it will be deployed at some point.

Initially, though, we’re baffled. Are Ashton and Reece friends, lovers or actors rehearsing scenes? Slowly we realise that they’re playing out past exchanges from their respective lives, each complaining when the other gets a line or an intention wrong.

These moments have the hectic gabble of a read-through, the characters exaggerated, with little emotion behind them apart from annoyance. Ashton, too, obsessively recreates lost maps, “to symbolically articulate where we are in space”, and gets shirty when Reece points out that he’s missed a street or two. The performances of all the actors are unremittingly blazing-eyed and shouty in this tiny venue.

There’s a teeth-itching sense of vagueness and stasis about the whole thing. What form did the apocalypse take? Why and how did Ashton build the bunker? Reece regularly ventures outside, so why don’t they leave? Why do Presley and Reece bang on about going to the seaside for an ice cream if they know the world is blasted?

I am normally a huge fan of Stephens, and there is fine writing here, especially in a single speech that marks the major, jack-in-the-box surprise of Ryan’s production (a second surprise can be seen coming a mile off). But there is also a sense of archness and artificiality to the dialogue and the plot, such as it is – of random elements thrown in to trip the audience up again.

At one point Ashton seems to fall seriously ill, then recovers. At a moment of high tension, Reece becomes deaf or distracted, responding to Presley only with the repeated word “what?” Mechanical clatter intrudes, then abates, and is never explained.

Maybe I’m being over-literal. Maybe this fractured, looping doomsday scenario is a legitimate response to a world that seems to be going to hell in a handcart (or perhaps in a MAGA hat). Maybe survivors will regress into comfortable memories and familiar desires (while Reece craves ice cream, Presley says he misses blowjobs).

But the dynamics of A Slow Fire feel too familiar, the execution too wilfully obscure. One of Stephens’s lines that I wrote down is: “I don’t have to understand the words, I just have to say them.” Well, quite.

To 26 Sept, jermynstreettheatre.co.uk