Celtic chant is the liturgical plainchant repertory of the Celtic rite of the Roman Catholic Church performed in Great Britain, Ireland and Brittany. It is related to, but distinct from the Gregorian chant of the Sarum use of the Roman rite which officially supplanted it by the 12th century. Although no Celtic chant was notated, some traces of its musical style are believed to remain.
The Celtic Church goes back to the Irish monastic traditions established by St. Patrick in the 5th century. The churches in Ireland and Great Britain had no central authority, and developed local traditions until Augustine of Canterbury and others imposed Benedictine monasticism and a version of the Roman rite starting in the 7th century. Notable in this transition from local Celtic customs to more standardized Roman traditions was the conflict over the dating of Easter, where the Roman tradition of solar dating finally supplanted the Irish lunar dating at the Synod of Tara in 692. Over the next several centuries, versions of the Roman rite such as the Use of Salisbury were gradually enforced in Brittany in the 9th century, Scotland in the 11th century, and in Wales, Ireland, and England in the 11th and 12th centuries.
The Irish monks famously established monasteries throughout Europe. As a result, Celtic chant was influenced by Spanish, Gallic, Roman, and Eastern traits. However, it shows the greatest liturgical similarity with Gallican chant.
Celtic chant was largely supplanted before being notated, and no musical specimen of Celtic chant prior to Roman influence survived, but possible traces of Celtic chant remain. One chant typical of those that may reflect Celtic style is Ibunt sancti, whose use was attested in Ireland. The original text shows such typical Celtic elements as alliteration and a couplet structure. The surviving melody, from a French manuscript, has an ABA structure, in which the opening phrase is repeated at the end of the melody, and the whole melody is repeated for the second half of the couplet. Neither the ABA structure nor the repeated melody for the couplet are typical of the Roman chant traditions, except in Sequences, which themselves trace back to Notker of St Gall's and Tuotilo's tropes at the Irish-founded Abbey of St. Gall.