Many people confuse the terms "Irish" and "Celtic." Irish music is a type of Celtic music, but not the only type. Irish music is, of course, from Ireland. "Celtic" can refer to Irish, English, Welsh, Manx and even some French.
The Celtic people groups had established themselves on the island of Ireland by 200 BC. Songs and tunes have been passed down from generation to generation by oral tradition. Songs composed by blind harpist O'Carolan date from the 1600s.
The harp is one of the cultural icons of Ireland, and was used extensively in the Middle Ages. The fiddle and the penny whistle are well-known for their use in traditional Irish music. The bodhran is a goat-skin drum that adds percussion to the mix. Concertinas and accordions are popular as well. Bagpipes are generally considered Scottish instruments, but there is an Irish pipe--the Uilleann.
There are two distinct styles of Irish music--dance and traditional--with several subsets. Dance tunes include the jig, reel and Irish polka. Many traditional songs are sung by groups with accompaniment. The purest form of traditional Irish singing is the sean-nos style. This is sung by a soloist, with little or no accompaniment.
Some modern groups fight to keep Irish music traditional; others blend the old and new to create Irish rock and other alternative styles.