In 2015, Lisa Hannigan made her film debut, providing the voice for a Selkie – that is to say, a mermaid – in the Oscar-nominated Irish animation Song of the Sea. Anyone who knows her music will recognize this as an inspired bit of casting. The sea recurs constantly as an image in her work, a great unknowable that can drown us or can bring us together, as a songwriter, she has the mermaid-like ability to go to places where, strictly speaking, we shouldn't be able to go. Hannigan first came to light as an angel-voiced, somewhat mysterious figure singing harmonies alongside Damien Rice. They played together for seven years, but it wasn't until the release of her solo debut Sea Sew in 2008 that the full spectrum of her abilities became apparent. A joyous kaleidoscope of love songs, laments, sea shanties and glockenspiels, with a cover hand-stitched by Lisa herself, the self-released album was a runaway success, nominated for a Choice award in Ireland and a Mercury Music Prize in the UK. The ecstatic reception gave her the confidence to explore deeper, darker emotions. Passenger, which came out in 2011, centred around the pain of leaving and the sense of apartness consequent to a life spent in constant motion. If Sea Sew bobs beautifully on the surface, you could say that Passenger explores the undercurrents of passion and heartache beneath the waves. Since then, on top of her acting duties, she's contributed a haunting cover of 'Danny Boy' to the TV series Fargo, as well as vocals for Steve Price's scores for Gravity (which won an Oscar) and Fury. And she's been writing At Swim, her third album, arguably her most bewitching collection yet. From the very start – autumn-tinged 'Fall', co-written with producer and Renaissance man Joe Henry – it crackles with an eerie Hallowe'en magic. The song begins with an acoustic guitar and a whispered invitation: 'Hop the fences, steal the cars/Run on fumes and from the law/And burn for us right through the fall...' As the voices and the guitars chime in, we're up and running, off on an outlaw journey. With every song that follows we travel deeper into a darkly magical space where the boundaries between love and death, past and present, grief and happiness, are dissolved. Hannigan's never been a singer who's afraid of the dark, but here she handles her themes with an empathy and directness that can only come with experience. So it's a surprise to learn that the record didn't come easily, at least not at first. 'I toured with Passenger for nearly two years,' Lisa says. 'And then I came back and sat down to write the new one. But this time I found it really hard.' One reason was that, because of a new relationship, she was dividing her time between Dublin and London. 'London's such an amazing city in many ways, but all of the clichés you hear about it are true,' she says. 'It's very big and very alienating. I was away from home, away from my usual anchors and points of reference, and I felt a bit adrift and lost.' Used to having musician friends to bounce ideas off and test new songs on, she now found herself writing on her own – or not writing. Landlocked? 'You could say that,' she laughs. 'And it's kind of a vicious circle. The longer you're not writing, the larger it looms.' The breakthrough came when she got an email out of the blue from Aaron Dessner, guitarist with The National and producer of the bestselling Dark Was the Night, who has also produced recent albums for This Is the Kit and Frightened Rabbit. 'He'd seen a clip of me singing and he wrote to say, “Hey, if you ever want to do something together...” And of course I wrote back straight away: “Yes!” So he started sending me these pieces of music, and I'd put them on and record myself humming along into my phone, and suddenly all of these songs started appearing.' 'Lo' was the first song they wrote together – a hypnotic ode to insomnia that starts in a state of small-hours dread but arrives at a state close to exaltation. Listening to it, it's as if you can hear Hannigan's heart lift as she works out how to confront the darkness that's been oppressing her, take hold of it and transform it into something else. Working with Dessner, rediscovering the collaborative spirit she'd missed in Dublin enabled Lisa to see her life in London in a different light. So while At Swim is in part about homesickness and isolation, it's also – profoundly and very movingly – about love. 'I was adrift and caught in the ropes/Under a pinhole sky, blowing off course...' begins 'Ora', a gorgeous aquatic ballad worthy of her mermaid alter ego in Song of the Sea. 'Aaron sent me the music, just the piano,' Lisa says. 'And the way he played it immediately conjured up in my mind the heave and the haul of oars on water. Next thing the words and the melody had just arrived in my head.' From that sense of lostness and uncertainty comes a new hope, and a fragile, ravishing dream of love. 'You'll be the boat and/I'll be the sea,' it closes. 'Won't you come with me?' Who could resist? 'A Prayer for the Dying' was inspired by the death of a friend's parent after a long illness. 'They'd had a very long and happy marriage and the loss was devastating,' Lisa recalls. 'I wanted to try and express that grief but also to pay tribute to their marriage.' Hence the song's passage from Patsy Cline-esque lament to its starlit, shimmering chorus: 'Your heart, my heart' - four words that here somehow convey all of the power of a whole anthology of love poetry. If At Swim finds love in darkness, however, there's also a darkness to the love. In 'Snow', love is enough to make the physical world disappear: 'you were the snow falling down...I was the city losing colour and sound'. But there's something suffocating about that image of the lovers buried in each other, 'sunk like treasure'; and wherever it is they're travelling to, it's with the knowledge that 'we would never be here again'. And how about 'Tender', with its twisting dancers, and that sinister, unexplained loose tooth? The same ambiguity suffuses album closer 'Barton', another song written with Dessner, and marked by a soaring vocal performance amid a shower of guitar harmonics and fizzing drumbeats, the latter provided by the National's Bryan Devendorf. 'Broken as it is, this is a love,' Hannigan sings, and we realize, if we haven't already, that we've come some distance from the wide-eyed whimsy of Sea Sew. Dessner and Hannigan finally met in Denmark, some months after they'd begun emailing, and began to map out the album. The following September, she travelled to Hudson, New York, where Dessner lives. 'It's upstate New York, proper rural, with sheep and chickens on the side of the road.' It's also home to Future Past Studio, a huge converted church where, with Dessner on guitar, Logan Coale on bass, and Ross Turner (of I Am the Cosmos) on drums – most of the album was recorded in just 7 days. Then, in the weeks that followed, Dessner continued to work on the music in his home studio. 'He didn't want it to sound too pretty,' Lisa says. 'He wanted it to have a texture rather than have big arcing melodies.' Hence the strange sounds that lurk beneath the surface of the album, and strokes of idiosyncratic genius like the trombone chorale that plays the funeral horns at the end of 'We, the Drowned', and the strings – brilliantly arranged by German composer, conductor and erstwhile Gorillaz collaborator André de Ridder – that lean slightly askew throughout. Were you both on the same page throughout the recording? 'I think Aaron would have liked a few more guitar solos,' she laughs. 'But whenever he'd suggest it I'd say, “Hmm, maybe we should have a violin instead.”' (The album does feature Lisa's first-ever banjo solo, however – on “Undertow”, an addictive, propulsive number about sex and desire.) For her part, Lisa wanted the record to feature lots of vocal harmonies. 'Because I started out doing harmony with Damien, I shied away from it in my own records. I wanted to show I could do other things. But with this one I went for it.' This is nowhere clearer than on 'Anahorish', an a capella reconfiguring of a poem by Seamus Heaney. Otherworldly and quite breathtaking, the result is like a mythic Irish version of the Sirens' spiritual in O Brother Where Art Thou? Heaney – a famous music-lover who died while the album was being written – would surely have approved. And then, second from the end, there's the towering 'Funeral Suit'. Apparently a simple song about a night in, it pulls together all the themes of the album – love, death, London, homesickness, 'the past and future tense', isolation and togetherness and the long hard struggle to belong. 'He came by in his funeral suit/In an open-hearted shade of blue...' 'That song's completely autobiographical in that himself came back from his nana's funeral very early on in our relationship and I remember we were supposed to be going out somewhere in London and I didn't know where anything was,' Lisa recalls – a state of affairs beautifully captured in the lines: 'To Bermondsey or to Shoreditch?/I said I don't know which is which/The night a thread for him to stitch/for me the unbelieving...' In the end, they stay home, and drink gin, 'and dance around this borrowed kitchen...' And from here the song builds to an extraordinary wordless chorus (mention must be made of Coale's sublime basswork here) that expresses everything that's going on under the surface. Because of course, it's on seemingly ordinary nights like this that whole lives turn; it's on nights like this that you fall in love. And that's the thing about Lisa Hannigan. She'll tell you she's off course, or unbelieving, or at swim, and then she'll turn around and write a song so direct and so natural and so perfect it seems like it's always been there. The truth, one suspects, is that she feels quite happy being lost at sea. At this point in her career, she's a strong enough swimmer to go as far out as she wants, right out into the storm; she's brave enough to plumb the darkest depths, down to where the brightest treasure lies. And she's great-hearted enough to bring it back for us.