Planxty were an Irish folk music band formed in January 1972, consisting initially of Christy Moore (vocals, acoustic guitar, bodhrán), Andy Irvine (vocals, mandolin, mandola, bouzouki, hurdy-gurdy, harmonica), Dónal Lunny (bouzouki, guitars, bodhrán, keyboards), and Liam O'Flynn (uilleann pipes, tin whistle). They transformed and popularized Irish folk music, touring and recording to great acclaim.

Subsequently, Johnny Moynihan, Paul Brady, Matt Molloy (flute), Bill Whelan (keyboards), Nollaig Casey (fiddle) and briefly, Noel Hill (concertina) and Tony Linnane (fiddle) were also temporary members.

Planxty broke up twice, first in December 1975 and again in April 1983. The original quartet reunited in October 2003 and their final performance was on 31 January 2005.

Christy Moore and Dónal Lunny had been friends since school days in Newbridge, County Kildare, Lunny having taught Moore how to play both guitar and bodhrán. Before the formation of Planxty, Lunny had been playing in a duet with Andy Irvine after the latter's return from Eastern Europe and they had also launched their own folk club, downstairs at Slattery's, called The Mugs Gig. Liam O'Flynn was playing in public and on the radio, and was well respected in traditional folk circles. All members were familiar with one another's work to varying degrees but were first brought together during the summer of 1971 to record Moore's second solo album, Prosperous, at his sister's house, in the village of the same name.

In January 1972, the four joined forces to form Planxty, recording their first single, "Three Drunken Maidens"/"Sí-Bheag, Sí-Mhór", in Trend Studios on 18 January 1972. The band performed on RTÉ's The Late, Late Show the following Saturday, 22 January 1972, and played their first show on 6 March 1972, a 30-minute set at The Mugs Gig on a bill that included balladeer Paddy Reilly. They then assumed a weekly residency at The Mugs Gig, began rehearsing, and started playing live around Ireland.

The group's first major performance–opening for Donovan at the Hangar in Galway, at Easter 1972–was a huge success. Neither the audience nor the band knew what to expect, and both were pleasantly surprised. Irvine, unable to see the audience through the glare of the stage lights, was worried that the crowd might be on the verge of rioting. It took him several minutes to realize that what he was hearing was the expression of their enthusiasm. A rough quality recording of the song "Raggle Taggle Gypsy" from this concert was included on the 2004 retrospective, Christy Moore – The Box Set: 1964–2004, complete with the audience's reaction.

Planxty's first single, "Three Drunken Maidens", was released by their manager Des Kelly's label, Ruby Records, reaching no. 7 in the Irish charts. The next single, a re-recording of "The Cliffs of Dooneen", previously recorded for the Prosperous album, made it to no. 3. Two full albums followed: Planxty, recorded at Command Studios in London during September 1972, and The Well Below the Valley, recorded at the Escape Studios in Kent, from 18 June 1973.

The group's increasing popularity led to heavy touring throughout Ireland, Britain, France, Germany, Italy, Spain and northern Europe.