Carlingford Lough is a fjord that separates Northern Ireland and the Republic of Ireland. It has a ferry which, twice an hour every hour, crosses what was once one of the tensest borders in the world. It’s a border that today has an almost surreal existence as something that some people believe exists, and should exist, and that some people believe doesn’t and shouldn’t.

The main setup of the film is candid filming into people’s cars as they make the journey on the ferry across the lough. The framing is always the same: looking straight through the windscreen at whoever’s in the car, static and unmoving. The film-makers travelled back and forth on the ferry every day, and on each crossing asked someone if they could film their journey. They attached a camera to their bonnet, mic’d up the interior of the car, and then just listened in on their interesting conversations. The result is brilliant, and surprisingly unfiltered.

We overhear people talking about going shopping. Filing their nails or applying makeup. Taking the dogs to the beach. Discussing Brexit and the border. Admiring either the Mourne mountains or the Cooleys, depending on the direction of travel. Travelling northwards, travelling southwards, travelling away from home and travelling back again.

Whatever else they’re doing, most of them are also on the lookout for Finn the Dolphin, who’s been spotted swimming alongside the ferry and who ends up as the film’s… well, its white whale.

The journeys where people presumably said nothing interesting, and so we just see shots of them staring blankly forward, are almost as interesting: a varied cast of characters, contemplating this strange place.

Perhaps realising that it would be a bit monotonous to stay on the ferry the whole time, it follows people beyond it, so we see where they’re going too – whether that’s retirees playing golf, a Gaelic football team playing a match, or oyster farmers hauling up their harvest.

Over everything hangs the dread spectre of Brexit, which at one point threatened to put a hard border down the lough complete with customs checks – something that would surely have upset the two decades of peace since the Good Friday Agreement.

So we hear from Unionist Brexit Dad, who says “we voted to leave, so now our home affairs aren’t dealt with in a foreign country”, and who explains to his son that, while it’s good that he can now play with Catholics (“when I was a boy I didn’t even know what Catholics looked like”) it’s extremely important to remember that he’s British, and will always be British, in a way that seems to be aimed more at the camera than his (slightly bewildered) son.

And we hear from Nationalist Irish Gran, who is unshakeably Irish and who dreams of living in a United Ireland, who wouldn’t be British “even if they took over the whole world”.

For both of them, Brexit has the potential to confuse old certainties and reopen old wounds. “They butchered, they bombed. They murdered friends of mine. They attempted to murder myself. And I’m meant to go into their way of thinking?” says Unionist Brexit Dad. “It won’t happen!”

A group of Filipinos, sat in their car, simply can’t see what all the fuss is about. “The Northern people and the people from the Republic are the same. Because they are all Irish, right? The person we were talking to was Irish… he’s from there, but he lives here. Their behaviours are all the same. For me, they’re all the same.”

The film ends with a lone man in a car, the characteristic Irish rain bucketing down, looking bored as orchestral strings begin to stire. But, out of nowhere, he opens his mouth and sings: it’s Comfort ye my people, from Handel’s Messiah.

Comfort ye, comfort ye My people, saith your God. Speak ye comfortably to Jerusalem, and cry unto her, that her warfare is accomplished, that her Iniquity is pardoned. The voice of him that crieth in the wilderness, Prepare ye the way of the Lord. Make straight in the desert a highway for Our God.