Greta Garbo Came to Donegal, Tricycle Theatre, London

Frank McGuinness's emotionally rich and highly enjoyable new play has a Chekhovian structure. As in Uncle Vanya, an outsider descends on a provincial community, throws the proverbial cat amongst the pigeons and then departs, having caused changes for good and ill. McGuinness is not the first Irish writer to work humane and tragicomic variations on this formula. The unique selling point of his new play, which is premiered in Nicolas Kent's atmospheric and beautifully acted and designed production at the Tricycle, is a fresh sexual and cultural twist.

The temporary guest is a sapphic Swedish superstar of the silver screen, and she is not the only character who – the period is the late 1960s – has to wrestle with a sexuality that is only gradually becoming mentionable, let alone acceptable. Priest-ridden Ireland is scarcely leading the way on this front. Sergeant Pepper is about to hit the shops, but for some the times, they aren't a-changin'.

Greta Garbo, for it is she, touches down in Donegal with a view to buying the Big House owned by a gay English painter (Daniel Gerroll) of effete, cravat-and-espadrilles, public-school provenance. A sexy young ex-boxer from south London (Tom McKay) does more with his master's body than merely guard it and – having once murdered a man in an excess of professionalism – is sometimes allowed to wear the pants in the partnership. Also, occasionally causing Garbo to mislay, if not lose, the control she so values and needs, there is the housekeeper, Paulie, who is brilliantly portrayed by Michelle Fairley as a woman whose huge capacity for sexual and familial love has been frustrated.

With stage presence to squander, a low-key imperiousness that operates to its own timetable of unthinking entitlement and a drop-dead witty delivery of direct, tact-free home truths, Caroline Lagerfelt is wonderfully good as Garbo. In fact, she is more striking than Garbo, whose eyes never brimmed with such elusive, hard-won humour.

McGuinness has hit on a very clever and fruitful conceit. As well as being a catalytic figure, the screen star is a dogmatic propagandist for values that throw into relief the culs de sac of sentimentality and dated political and religious allegiance that the Donegal denizens scurry down for protection. With a firmness that would make Catherine the Great look a tad indecisive, Garbo believes the human lot to be only partially remediable. As with the Stoics, this is expedited partly by a lowering of expectation.

The play is full of artfully angled and emotionally convincing contradictions. The housekeeper partly breaks through the star's defences and for all her talk of proud adjustment to circumstance, Garbo does set in train a benign scheme that will allow the housekeeper's niece to break free from her miserably co-dependent parents (played excellently by Angeline Ball and Owen McDonnell), who rely on her as glue, and go to Dublin to study medicine.

Uncle Vanya springs to mind forcibly at one point. The hero of that play haplessly takes two shots at his enemy, the professor, and misses with both. There is an equivalent moment here when the niece's drunken father fires a revolver – which he would often like to aim at his wife – into the air and, to his farcical surprise, finds that it is loaded.

But this is Ireland, where history has planted many an unexploded bomb in the path of posterity. Robert Jones's enchanting design elides the interior and exterior worlds, inundating the proceedings with blue sky. It is the same with McGuinness's art here – a case of "let it be" and "let it breathe".

To 20 February (020 7328 1000)