I’d never heard of it before, either — not the term (it’s also used for monuments on Malta, I believe) but the Celtic origin of this one.
I learned about it in searching for the root languages of Europe. Celtic was one of them. But, the Teutonic, Greek and Latin languages survived in Europe and the Celtic language was forced to migrate and thereby disappear from the continent.
I've also visited Ireland a couple of times and heard "the Irish." It doesn't sound like ANYthing I had ever heard before.
But, I felt that way when I was in Serbia and attended a Mass (Roman Catholic, of course). The only thing I could understand was their version of Jesus Christ. The rest was jibberish to my ears.