Keith Fay joined the technical death metal band Crypt in 1991 where he was introduced to John Clohessy who was the bass player of Crypt. They enjoyed local success but Keith wanted to experiment with folk music within metal music. Keith then formed a Tolkien-inspired black metal band by the name of Minas Tirith later in 1991. This band eventually became Cruachan with a demo recording distributed in 1993. Keith Fay ;credits the Irish rock band Horslips as a "huge influence on Cruachan," further noting that "what they were doing in the 70's is the equivalent of what we do now."

The year 1995 saw the release of Cruachan's debut album Tuatha na Gael, an album that "suffered under poor production." On the strength of this album, the band received attention from Century Media Records. The band found it amazing to receive interest from "such a big label" but were dismayed at the terms and conditions of the offered recording deal. After lengthy negotiations failed to yield a deal with Century Media, the band disbanded in 1997.

Cruachan reformed in January 1999 and after signing a recording deal with Hammerheart Records, they released the second album The Middle Kingdom in 2000. By this time, the band had dropped the black metal style in favour of a more traditional metal sound. They also expanded their roster by inviting guest vocalist Karen Gilligan to be a full-fledged member of the group. Their subsequent album Folk-Lore was co-produced by Shane McGowan of The Pogues fame. Shane McGowan also contributed vocals to two covers of traditional Irish songs, "Spancill Hill" and "Ride On." The latter was released as a single and the band experienced a small taste of commercial success when it entered the Irish charts.

The band released their fourth studio album Pagan in 2004 with cover artwork by famed Tolkien artist John Howe. The production on the album was subsequently criticized by frontman Keith Fay as "terrible". In 2005, the band parted ways with their label Karmageddon Media on good terms. For the band's fifth album, Cruachan returned to the same Sun Studios where they had recorded their first three albums. The result was The Morrigan's Call, an effort that Keith Fay described as their "best work to date, in every aspect." The album was released in November 2006 through AFM Records. Drummer Joe Farrell left the band shortly thereafter due to "personal reasons" and was replaced by Colin Purcell.

A further line-up change occurred in December 2008, as vocalist Karen Gilligan left the band "on good terms for various reasons." Keith Fay announced that he will take over full-time vocal duties.

A demo was recorded in 2010 which showed the band's return to a much harder more black metal influenced style. This demo was sent to specific record labels only and Candlelight Records UK signed them in June 2010. "Blood on the Black Robe", the first of the "Trilogy of Blood" albums was released in 2011.

2014 saw Cruachan leave Candlelight Records and sign with the dedicated folk and pagan metal record label, Trollzorn Records. This has also been the busiest year of the bands career with very high profile shows including a tour of Russia in March. Headline shows in Germany, Ireland, Ukraine and Israel, as well as appearances at Ostrov festival (Russia), Bandershtat festival (Ukraine), HoernerFest (Germany), Brutal Assault (Czech Rep.), Rock for Roots (Germany), Halloween Fest (Italy) and Eindhoven Metal Meeting (The Netherlands). In between these shows they were recording their next studio album. "Blood for the Blood God" was released in December 2014. Due to these recording commitments they had to turn down a place on Paganfest USA 2014.