Celtic music
Celtic music refers to the traditional or folk music that originates from countries or regions that, until a few hundred years ago, used Celtic languages. These include, especially, Irish traditional music (played throughout the island of Ireland and countries of the Irish diaspora) and Scottish traditional music (of Scotland as well as Cape Breton Island in Canada), but also Welsh traditional music (from Wales), Cornish traditional music (from Cornwall in England), Manx folk music (of the Isle of Man), Breton folk music (from Brittany, in the northwest of France), and Galician folk music (from Galicia, in the northwest of Spain). While Northumbrian traditional music is perhaps not strictly Celtic, the music of Northumbria--in the extreme north of England--shares affinities with Scottish music.
Scholars have not reached any firm conclusions about genetic relations between the traditional musics of Ireland and Scotland, on the one hand, and Brittany and Galicia, on the other. The music of Ireland and Scotland is perhaps closest of any, due to well-known (and ongoing) cross-pollination between the traditions. Many Irish reels originated in Scotland, and many Irish jigs and other tunes are played in Scotland. The Donegal fiddle tradition represents the coming-together of these two broad traditions.
It is extremely unlikely that modern Celtic music bears any relation to ancient Celtic music. Most of the tune types and instruments were created in recent centuries.
There is, however, a whole modern-day brand of "folk music" that uses the imagery and mythology of pagan Celtic culture. This is one sort of music that goes under the name "Celtic music" but is not specifically locatable. This may be thought of as a musical analogue to the revival of pagan religions.