Apart of the use of gaelic in Ireland, scottish isles and some parts of Wales, there's also a gaelic language spoken in French Brittany
Actually, this is one of my pet peeves. They don't speak Gaelic in Wales or in Brittany. I have to correct people on this point frequently.

Gaelic is one of several Celtic languages (Just like English is a Germanic language, and Spanish is a Romance language, etc.).

There are two main branches of Celtic languages, Q-Celtic and P-Celtic. Unless I have the reversed in my head (which is very well possible!), the Q-Celtic branch consists of the Gaelic languages spoken in Scotland and Ireland, and the Isle of Man. P-Celtic would consist of Welsh, Breton, Cornish, and more than likely the lost language of the Picts.

So the languages spoken in Scotland and Ireland, and the languages spoken in Wales and Brittany, can said to be *related* in the sense that they are both Celtic languages, but they are not the same language, and in fact Gaelic belongs to a completely different branch of the Celtic languages than do Welsh or Breton.

Aye,
Matt