It’s an unusual place, this Redwater (RTÉ, Sunday, 9.30pm), a small Irish seaside town festering with secrets, which still comes together to celebrate the Spring Equinox with surprising gusto.

It’s a big deal, this holiday, the residents tell Kat (Jessie Wallace) and Alfie (Shane Richie), the adorable EastEnders couple who have ended up west in this spin-off, in search of answers.

In real life, though, the Spring Equinox was long ago rebranded as a modest cultural event, known colloquially as St Patrick’s Day, of which there is no sign or mention. Instead, the town engages in a fancy-dress fun run, thronging the streets with carnival games and food stalls; displaying all the diversions you ordinarily associate with a village fête.

Kat and Alfie may have come to Ireland – from Spain, to where they were banished and for which they are better dressed – but it looks like a very British idea of Ireland.

"We thought we'd burn an outsider in a big wicker goat," deadpans the enjoyably guarded Fionnuala Flanagan, as suspicious matriarch Agnes, which would seem more intimidating if it wasn't such a culturally reassuring film reference. Kat is searching for her long-missing son, taken from her at birth and believed to be in Redwater, Waterford, but her quest is slowed by native distrust and – more obstructing still – a minefield of Irish clichés.

Alfie must push his broken-down taxi gently to its destination, for instance, in the first sign of charming, Ballykissangel-grade backwardness. Elsewhere, he is accepted by the locals by downing his Guinness in one gulp, while Kat ingratiates herself with whiskey chasers and a lusty appreciation of Irish grandiloquence.

"I love the way you talk," she tells Ian McElhinney's honey-tongued grandfather, Lance, a man who travels everywhere by horse in a costume last seen on Clint Eastwood.

There is, of course, a dark and sordid history beneath this strange vision of an Irish community. In 1997, EastEnders went offsite to depict Ireland as a land of marauding livestock and drunken louts, for which the BBC later apologised.

It’s hard to imagine anyone taking offence from Redwater’s caricatures, though, which do come with some self-effacing irony. Instead, the question is whether you can properly invest in Kat and Alfie, whose migration from Albert Square to Redwater is not quite as challenging as their migration from soap opera to drama.

Soaps ration out plot implausibilities over time: in Kat and Alfie’s case, affairs, bankruptcy, imprisonment, baby snatching, house fires, a lottery win – you name it. But here they come in a daft rush, basted in solemnity. “I know I’m not going to find him in the first 10 minutes,” Kat insists, convinced that Peter Campion’s mop-topped Andrew is her son within the show’s first 15 minutes.

Meanwhile Alfie, contending with a brain tumour, is assailed by visions, premonitions and perils that are hard to differentiate. Roused from an uneasy dream, he finds himself standing at a literal cliff edge, later rescued from another one by the show’s most unlikely character. This is a 33-year-old bearded hipster priest named Dermott (Oisín Stack), who accessorises his collar with a hoodie and a donkey jacket. For reasons yet unspecified, Dermott flies into a rage at the sight of orange juice, but the episode holds more upsetting discoveries for him than citrus.

In Dermot, the lightly pagan fascination with Spring Equinox settles into a much more recognisable idea of Irish people warped by religion – a cliché with some heft. Shot in muted hues and pale light which make its real location, Dunmore East, look especially serene, it is hard to decide whether Redwater is a silly show redeemed by its seriousness, or a serious show alleviated by its silliness. It is a peculiar crossover, somewhere between Britain and Ireland, soap and drama; but, so far, it is neither here nor there.