In the days after Cahill’s passing, still reeling from the loss, Hayes would pen a heartfelt written letter to Cahill which struck a chord with all who read it.

It was a profound and gently lyrical tribute to a man who had made a mark like no other on Hayes’s life. But it was also a universal expression of the deeply felt closeness that can often go unspoken in male friendships.

The letter began:

“Dear Dennis, we knew each other for over 35 years without ever really giving verbal expression to how we felt about each other.

“Once in a while, after a whiskey, you might lean your arm on my shoulder and tell me that I was a good guy, or that we’d made a difference. I knew what you meant.

“It would be an understatement however to say you didn’t like any mushy emotional talk – but yet, without ever having to say it, we were both keenly aware of our deep loyalty and love towards each other.

“That feeling of trust was always there in such a way that we never had to talk about it. We really were great friends, we loved each other, and we really had a hell of an adventure.”

Over the course of their long careers the two men had performed across the world, reinvigorating the spirit and expression of traditional Irish music, in the United States particularly – and their temperaments and musical styles seem to complement each other perfectly.

​They came, Hayes noted, “from different musical worlds”. Hayes had grown up in east Clare, on a small farm on the side of of a mountain. The county is, of course, synonymous with traditional music and Hayes’s father would have “thought of himself as a musician first and a farmer second”.

“I was drawn very early into the soulfulness and depth of the music I heard played around me,” he says. “I felt connected with an old world of feeling that the musicians knew was part of the music. And so, from an early age – around 13 – my number one mission was all about accessing that feeling through music.”

At that age he won his first of six All-Ireland Fiddle Competitions, and soon joined the Tulla Céilí Band – which his dad PJ and uncle Paddy Canny had founded in 1946.

​Thousands of miles across the ocean a kindred spirit had already begun the same process. Cahill, who was just two years younger, was born in Chicago – but both of his parents came from Baile na nGall in Co Kerry, and his mother, Anna, was a relative of Thomas Ashe, a founding member of the Irish Volunteers.

Like Hayes, he discovered music in childhood and began playing guitar at the age of nine. After completing his schooling he attended Chicago Musical College to study classical guitar. He soon took his talents out into the field, gigging at venues around Chicago and embarking on a career as a singer-songwriter during the city’s version of the folk revival.

Ireland’s own folk revival of the 1970s was overstated, Hayes says, and despite a certain revisionism in the decades since then, “it didn’t penetrate the mass public very much... it was still a niche thing.”

He worked as a jobbing musician, but never lived in Dublin. In 1985, when he was 23, he moved to Chicago and became active in the city’s traditional music scene. He first met Cahill at Fox’s Pub – both men lived nearby.

​“My first impression of Dennis was that he seemed kind, of, well... enigmatic, you could say,” Hayes now recalls. “Like, there was a dark veneer almost as if you couldn’t penetrate and get to know who he was. I got the sense early on that he was a very private human being and that he was guarded.

“I think he was protective of himself in some ways. But, obviously, as I got to know him, I realised he was an incredibly sensitive, caring, thoughtful human being.”

Cahill “did not know much about traditional music when I met him”, Hayes says. In a country in which people often list off their ethnic origins in response to the question as to where they’re from, Cahill was resolutely American. He never added the ‘Irish-’ prefix.

“He used to famously say: ‘I’m just from here [America] because someone’s got to be,’” Hayes recalls with a soft smile of remembrance. “But yet, despite identifying with America, he found himself surrounded by Irish people his entire life. And he also found himself remarkably at home here in Ireland.”

As their friendship deepened, the pair formed Midnight Court – a “wild and noisy” jazz/folk outfit, and lived together in a house that became “a little music hub of Chicago”. Both men were active in a music scene that featured people like John Williams, Liz Carroll and Michael Flatley – and the energy and verve of the playing drew in many others from the American folk music scene, including John Prine.

In 1992 Hayes moved to Seattle and toured up and down the western coast of the States, and Ireland, playing with various noted musicians, such as Steve Cooney and Randal Bays along the way.

But he wanted someone who would be free to travel just with him – and so he called up his “old buddy that I had left behind in Chicago”.

“Right away he said: ‘I’m in, I’ll cancel all my gigs.’”

The pair released their incredible debut The Lonesome Touch in 1997 and followed that up with Live in Seattle in 1999. By then their musical rapport – built up over years of talking about music, sharing recordings and absorbing each other’s influences – was rock solid, and critics marvelled at the duo’s incredible onstage chemistry when they seemed to commune as though nobody else was there.

The New York Times wrote that “stripping old reels and jigs to their essence, leaving space between the notes for harmonics and whispered blue notes, Mr Hayes and Mr Cahill created a Celtic complement to Steve Reich’s Quartets or Miles Davis’s Sketches of Spain.” 

Hayes and Cahill played with many music legends, including Sting and Sinéad O’Connor, and met Barack Obama at the White House in 2011.

Along with singer Iarla Ó Lionáird, fiddler Caoimhín Ó Raghallaigh and pianist Thomas Bartlett, they were also members of supergroup The Gloaming, who released their titular debut in 2014 and won the Meteor Prize for Irish Album of the Year.

“Nobody before you had ever played those chords and rhythms with Irish music, in the way you did,” Hayes wrote in his letter to Cahill. “You matched the beauty of these traditional Irish melodies with your own equally beautiful sequences of chords and hypnotic rhythms.

“Every night we played, we would give it our all, we’d zig and zag until we locked in. Some of those moments were sublime moments where our connection was truly telepathic. There were so many times on stage when you were simply able to read my mind – and I hope that you still can.”

​Cahill’s parents had died young and he had just one sibling, a sister. “He didn’t have a lot of family, but he became a member of my family,” Hayes says.

“Both my father and mother loved him. He was at home in that house back down in Co Clare, and he had a room that was just known as Dennis’s room, so he could come and go there freely.”

Cahill’s first wife, Gwen Sale, died in a tragic car accident in 2002.

“I remember after that I said: ‘Well, if you need to, take time, take time off.’ And he goes: ‘No, I’d rather be playing,’” recalls Hayes. “So he continued playing music almost straight away – because that’s where his comfort was. He didn’t want to lose music and a wife.”

At the Feakle Festival in 2007, Cahill met Mary Joyce, a New Yorker. They fell in love and married 11 years ago on his birthday.

“They married six or eight months after I met my wife,” Cahill explains. “So I kept the suit,” he adds with a wide smile.

Like his close friend, Hayes met his wife, Madrid-born Lina Pelaez, in Feakle – where he noticed her in the audience of the famous music festival. She had been living in Clare for many years. At the end of one concert, Lina went up to him and asked if he’d play at the opening of her Steiner-based pre-school, Brigit’s Hearth, near Scariff.

Hayes passed his email address onto her and they began a correspondence. Hayes had been “avoiding” marriage for years, he told People & Culture in 2018, but when she asked him outright if he was the ‘marrying type’ he said that yes he was – at least in the context of having met her. And so he asked her to marry her – at one of her favourite spots, under the Brian Boru oak tree in Tuamgraney.

They were married in 2011, on the anniversary of the day they met.

Both of Hayes’s parents have passed away and he says in his loss he identified with Patrick Kavanagh’s poem, ‘Memory of My Father’ – in which the poet sees glimpses of his own late father in passers by.

“I remember one day just seeing somebody in Madrid who walked just like my mother did. Same size, same height, same hair, same walk. I was going: ‘Oh my God.’”

He experienced a similar thing after Cahill’s death.

In the letter, Hayes wrote: “It was kind of fitting that I found myself in an airport for many hours yesterday – maybe we spent more time together in airports than anywhere else.

“We’d often meet at the gate of some connecting flight, you having come from Chicago and me from wherever I was living at the time. I’d be searching for that backward-slanting baseball cap in the crowd, or sometimes just your shaved head.

“Yesterday in the distance I spotted somebody from the back with a shaved head and a black T-shirt – and for an instant I had that familiar momentary jolt of thinking I’d just found you.”

He now remembers that moment.

“I got startled for a second,” he says. “It seemed like some version of him.”

“In reality,” Hayes wrote in his letter to Cahill, “I never knew anyone remotely like you. You were one of a kind, a very special blend of talent, humility, grace and good humour.”

​Perhaps part of the reason why Hayes’s letter strikes such a chord is the realisation that many of us, especially men, never tell the people nearest to us quite how much they mean.

But there was no need to tell Dennis how much he loved him, Hayes says. “Because he knew. And I knew he did too. So that didn’t feel like a lost chance. And the last time I saw him I did get to tell him I loved him.

When I ask how he has been coping with the grief, he responds: “I probably deal with it the way Dennis himself would deal with it – which is not to say much about it at all. Just to say: ‘Well, that’s it.’ But in reality it’s a very gradual process.

“Now and again I get these little jolts of feeling, like: ‘Oh my God, it’s really over.’ Like just the final realisation that it will never happen again. And that’s life. And that’s what the loss of such a close friend and colleague and partner means.

“I feel like I’ve lost a part of myself as well. I do think that a lot of the things I do now still are shaped and impacted by my whole life with him playing music, how I hear it, what I want to hear, how I imagine music.

“It’s also all been shaped, I think, very deeply by my years of experience playing with Dennis.”

The documentary ‘Dennis Cahill: Litir ó do Chara’ is on TG4 at 9.30pm on March 5

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