They have to learn Irish while in school (they call it Irish since there's other Gaelic languages like Scottish and Welsh). They do learn it, but once they graduate from school they don't really use it so most forget it. There are some areas where people grow up speaking Irish, so there are some native speakers of Irish, they're just not very common.
It's probably similar to what Vivienne describes in Scotland. Some people in some areas grow up using it, and the government is trying to revive the language in other areas.