Clannad, the Donegal family band that improbably became a worldwide pop music sensation, is calling it quits after more than 50 years, and will bid adieu to Boston on Sept. 22 at 8 p.m. at the Wilbur Theatre [thewilbur.com].

Sadly, Clannad’s “In a Lifetime” farewell tour has not been a smooth-running affair. It was originally supposed to take place in Ireland, the UK, and US between March and October of 2020 but got pushed back due to the pandemic. Then last October, guitarist/vocalist Noel Duggan died – six years after his twin brother Pádraig, who also sang and played guitar, mandolin and mandola, had passed away. Ultimately, the tour was split into UK/Ireland and US legs.

So, it will be the Brennan siblings, Moya (harp, vocals), Pól (flute, guitar, whistles, vocals) and Ciarán (double bass, guitar, keyboards, vocals) – Noel and Pádraig were their uncles – fronting the band when the curtain rises at the Wilbur.

Most retrospectives of Clannad focus on their 1980s hits, notably “In a Lifetime” – featuring Moya Brennan’s celebrated vocal duet with U2’s Bono – “Theme from Harry’s Game” and “Newgrange,” and their lush, reverb-accentuated, synthesizer-boosted sound often regarded as a prototype of “new age” music. The now-ubiquitous “Celtic” tag also became associated with them in some quarters, which made for pretty lively discussions among pundits and listeners.

Nomenclature aside, by the end of the 1990s, Clannad had become an international success, with record sales of more than 15 million, film and TV soundtracks, and several awards, including a Grammy.

But the 1970s Clannad was a significantly different creature. To be sure, the rock, jazz, and other modern influences were there, but their sound was predominantly acoustic and they played a far more traditional-oriented repertoire – many of their songs in Gaelic – with Moya Brennan’s captivating lead vocals and Irish harp in a very prominent role. Their showstopper was an extended version of “Nil Se’n Lá,” with each member taking a solo improvisation, including Pól on flute and Ciarán on double bass; among their other mainstays were “Siúil a Rúin,” “Two Sisters” and “Down by the Salley Gardens,” and instrumentals like “Eleanor Plunkett” and “Brian Boru’s March.”  Even in the early years, though, Clannad showed a predilection for going outside the traditional realm, what with a haunting cover of Bonnie Dobson’s “Morning Dew” and Padraig and Pól’s sprightly pop composition “Liza” – surely one of the earliest examples of Gaelic folk rock.

Yet in a 2011 interview with the Boston Irish Reporter, Moya noted that the band had its share of detractors at the outset. She, her brothers and uncles had been tremendously inspired as teens by listening to pirate radio stations – which they were able to access because they lived on the coast in Gweedore – and incorporated some of the styles and ideas they heard, including, for example, the harmony singing of the Beach Boys and The Mamas and The Papas. This did not go over particularly well at home.

“When we first began playing, we were shunned,” Moya recalled. “At that time, Gaelic songs were just never done with instruments – except the harp – or with harmony voices; it was sacrilegious. That’s why we headed off to Europe a lot at the beginning, because they didn’t care what language we sang in. We were determined to stick to our guns. When you’re true to something, you do it for years because you believe in it, if you’re honest.”

So the success in 1982 of “Theme from Harry’s Game” – written for the titular British TV series – was all the more sweeter: It was the first, and is still the only, British hit single (reaching No. 5 in the charts) to be sung entirely in Irish; several years later, “Harry’s Game” was used in a Volkswagen commercial in the US, where by then Clannad also had made a strong impression.

“Whatever we did, whether it was with traditional music or our own material, it all felt natural to us, it wasn’t a stretch,” Moya told the Reporter. “We wanted to enhance the music, and we did it with respect. We felt our audience would understand and appreciate that.”

Of course, the collaboration with Bono on “In a Lifetime” was a big factor in Clannad’s rise to popularity. As Moya recalled, they ran into him at a pub while taking a break from a recording session and were pleasantly surprised to learn that he was one of their biggest fans. This led to his appearance on the recording, and eventually to the making of the “In a Lifetime” video in Gweedore (Bono arrived for the filming in a hearse he had recently bought, Moya noted, and which was used in the video).

Clannad underwent a series of changes beginning in 1990 when Pól left to work with Peter Gabriel and from the late ’90s through much of the first decade of the 21st century, the remaining members pursued individual projects as well (although they did reunite for a special surprise concert to honor their parents, Leo and Máire). Pól returned in 2011, in time for the band to record its last studio album, “Nádúr” (2013), and do an international tour that finished up about two years before Pádraig’s death; that same year, Moya was diagnosed with a progressive lung disease.

Yet in 2020, they were ready to hit the road once again, in conjunction with the release of their retrospective album “In a Lifetime,” with two new tracks including their cover of Sandy Denny’s “Who Knows (Where the Time Goes)” – their first new recordings since Pádraig’s death. In February of this year, Clannad finally gave its last concert on Irish ground with a show in Dublin, and on Oct. 9 they’ll ring down the final curtain in Seattle.

Speaking with RTÉ Guide prior to the Dublin gig, Moya seemed at peace about the long goodbye: “Two of the band have passed away and we want to go out with a bang, rather than just dangle along for a few more years. It’s bittersweet but we’re ready for it. It’s nice to finally put Clannad to bed.”