If I remember my studies, the Gaelic has two main branches Cymric (welsh and breton), and Scots/Irish. There are quite a few disimilarities between the accents etc. I am not sure about where similarities between Irish Gaelic and Sanskrit may lie.
There are some Welsh that claim Prince Madog was the first European to travel to the Americas in the tenth century (I think) so it might be feasible that they had travelled to India too.
You forgot Iberian (Spanish) Galatian (French to middle-east) and I can't recall the Russo-Celts, but them as well.
Many of the words are pronounced similarly. My book on Celts is currently at my girlfriends house so I don't have all the info on hand, but words for common things sound ALOT alike.
I'll get back to you on some examples if you remind me later this week. (tomorrow should be fine)
Actually, remaining Celtic languages are divided into two categories, Gaelic and Brythonic. Irish, Max and Scots Gaelic are in the Gaelic category; Welsh, Cornish and the language spoken in Brittany are in the Brythonic. Gaelic is thought to be the more ancient of the two divisions.
This chart might help. According to it, Cornish, Breton and Welsh all derive from 'Brittonic', whereas Gaelic (Scottish, Irish, Manx) derives from Goidelic:
http://www.bartleby.com/61/indoeuro.html
Language origins are fascinating. I need to do more reading to catch up on recent ideas.