Irish is a Gaelic language and among the Indo-European languages. It came from Ireland and was the Irish people's first and most dominant language before English was introduced in the late 18th century.
The language is one of the most ancient European languages still around. Currently, it only has few speakers, and the reason why most people learn it is to prevent its extinction. When the Irish people traveled to other areas such as the Isle of Man and Scotland, they took it with them. This then gave rise to Scottish Gaelic, Manx, and other languages with a blend of Irish in them.