It was never going to be a profound meditation on the human condition, but David Ireland's sex comedy (directed by Game of Thrones' Conleth Hill) isn't shy with its allusions to bigotry, grief and the kind of snap judgements that create distance between people.

Stevie (Declan Rodgers), a Belfast lad in his mid-20s, has just been dumped and is embarking on a new fling with Karen Dunbar's 49 year old Glaswegian widow Martha. That's until his staunchly Protestant Mammy Dorothy (Carol Moore) and sister Rebecca (Abigail McGibbon) enter his pizza box-strewn lair to interfere, hurling labels at his would-be partner and slut-shaming.

The cast are routinely fantastic: Dorothy is a capricious pitbull of a woman, incandescently ranting against Martha's atheism, as Rebecca becomes a cheerfully brassy co-conspirator with her wee brother. The pacing is zippy and effortless; the volley of Stevie and Rebecca's well-meaning insults reinstate the truism that we revert to being children in the family home, and are as recognisable as they are wince-inducing. The tentative scene in the cafe where Martha and Stevie meet is tender and funny, displaying a warm, dignified side to Dunbar which is a real pleasure to watch.

However, in spite of a funny scene involving superhero role play, Can't Forget About You doesn't hit hard enough, and quickly slumps into post-coital lethargy, resorting to trite farce. Ireland could have explored the invisibility of older women and religious issues more, as he hints at the start. But he bottles it, choosing sentimentality instead.

Rebecca's attempt to speak Ulster-Scots to impress Martha becomes tiresome very quickly. Still, the anger between mother and daughter feels real enough, bubbling over into almost Mike Leigh territory, and Dunbar invests Martha with real subtlety and grace – even when teetering around in a ridiculous Wonder Woman costume and platform boots.