The name reaches across millennia. Grianan of Aileach - the Stony House of the Sun - is a hilltop ring fort at Burt on the Inishowen Peninsula, built by the Northern Ui Neill in the 6th or 7th century CE on what may have been a much older Iron Age site. The theatre that borrowed its name opened its doors on 4 October 1999, with its official opening on 12 November that year. The first show on the An Grianan stage was Magic of the Musicals - a perfectly serviceable opening, lacking the gravity the name might suggest. With 383 seats, the venue became and remains the largest theatre in County Donegal. It is run by Patricia McBride and serves drama, comedy, music, pantomime, family shows, workshops and classes to a region that had long lacked a dedicated cultural anchor.

Backstage, in the green room, hangs a framed T-shirt signed by Glen Hansard, the Oscar-winning songwriter for the film Once and frontman of The Frames. He declared the room the best room in Ireland. The Frames play An Grianan regularly. Sharon Shannon played here in 2005. Isla Grant returned to Ireland in February 2015 with a tour that stopped at An Grianan on the 10th. The world premiere of the Irish-language-titled but English-language musical Caisleain Oir happened on this stage in 2000. Comedians who have walked these boards include Tommy Tiernan, Dylan Moran, Dara O Briain, Jimmy Carr, Ed Byrne, Pat Shortt, David O'Doherty and, in May 2012, Ardal O'Hanlon - Father Dougal McGuire to a generation that grew up with Father Ted. Neil Delamere returns so often the audience knows the rhythm of his act.

An Grianan has hosted the Fleadh Cheoil na hEireann twice, in 2005 and 2006 - Ireland's premier traditional music competition, which brings tens of thousands of musicians and listeners to whichever town earns the year's hosting honour. It hosted the Pan Celtic Festival in 2007. It is the main venue partner of the Earagail Arts Festival each July, the Irish Aerial Dance Fest each June, and Letterkenny Trad Week each January. In May 2007, a graphic design studio based at Rathmullan in nearby Donegal won the European Design Award in Athens for the theatre's branding. In 2013, An Grianan was named IMRO Ulster Music Venue of the Year. In 2015, it won recognition for its disability access - real provision, not a tokens: designated auditorium seating, a lift, accessible WC, and a Sennheiser assistive listening system that operates via smartphone.

In 2024, An Grianan celebrated 25 years since opening with a gala and an exhibition. The story it told was a story Letterkenny had not been able to tell about itself before 1999 - that a town in the northwest could build and sustain a professional theatre, draw international acts, host its own youth productions of Bugsy Malone and Alice in Wonderland and a Vampire Story, and run a coffee house that doubles as a show-night bar. In December 2025, the building closed for renovations, scheduled to reopen in the new year. The exhibitions programme was paused. The youth theatre kept meeting on Wednesdays and Thursdays. The casual lift Sean Maloney offered Michael D. Higgins three decades ago had become a piece of municipal infrastructure that thousands of people in Donegal could not now imagine living without.