Accompanying Vocal music was the main role of the Gaelic harp. A harper would work with a singer, playing to accompany their performance of a complex formal Gaelic poem or song. Sometimes, especially later in the tradition, the harper would sing whilst playing.
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Instrumental music was played by the harpers. From the 17th century, the earliest notated Gaelic harp tunes are instrumental compositions, known as ports. Song airs were also played, sometimes elaborated and formalised.
Ceòl mór ('great music') is the modern term used to refer to the most formal and elaborate instrumental music of the Gaelic world. Now played only on the bagpipes, where it is known as píobaireachd or pibroch, this style of music is much older and used to be played on fiddle and also on Gaelic harp. A piece is generally formed of a slow air with figured variations and can last for 10 minutes or more.
The Gaelic harp was used for church music from earliest times to the end of the tradition, but little is currently known about the details. 18th and 19th century stories include accounts of harpers providing music for the liturgy.
Foreign music was played on Gaelic harps in Ireland and Scotland as well as abroad. Irish harpers worked in the courts of England, Spain and even Poland; the harpers at home didn't hesitate to include the latest imported styles and tunes in their repertory.