There are in fact SIX Celtic languages, which divide down into two groups of three........Group One :- Irish, Manx and Scotts Gaidhlig and GroupTwo :- Welsh Cornish and Breton.
The three in group one are classified as Goidelic languages and the second group are classified as Brythonic languages. This distinction is often referred to as "Q Celtic" (Goidelic) and "P Celtic"(Brythonic) because words that have a "Q" sound in group one have a "P" sound in group two .......an example being the word for "head", which in Irish is "Ceann" and in Cornish is "Pen" (I've often wondered if this difference might have had anything to do with the expression in English "Mind your P's and Q's" !!)
Anyway, it has been my experience as an Irish speaker(Munster Dialect) that I can follow with difficulty what is being said in Manx or Scotts Gaidhlig, but as my ear/brain "tunes in" the percentage that I can understand increases. However, being an Irish speaker gives me no advantage whatever when listening to either Welsh, Cornish, or Breton !!
As I now live in Cornwall and have Cornish speaking friends, I'm beginning to understand a little Cornish and while it is completely different from Irish, I can recognise the common Celtic root words.
In any given year I attend at least three Celtic Festivals and have picked up words and phrases in all six languages and I will give an example of a simple question in all six. The question "Where do you live?" becomes :
Cá bhfhuil tú id' chónaí?         in Irish
C'raad t'ou cummal?             in Manx
Cáite bheil thú a' Fuireach?      in Scotts Gaidhlig
Pelec'h emaoc'h o chom?       in Breton
Lie ydych chi'n byw?             in Welsh
Ple'th os ta trygys?             in Cornish