Celtic Language is a language family (Like Romance Languages)
So languages that are apart of the Celtic Language family are Irish (or Irish Gaelic), Scottish (or Scottish Gaelic), Welsh, Manx and Cornish.
(Like French, Italian, Spanish and Portuguese are apart of the Romance Language Family)
Early forms of Irish (Irish Gaelic) from oldest to most recent before it's current form are Primitive Irish -> Old Irish -> Middle Irish -> Classical Irish.
*~Loads of Randomness~*
My PNLs:
http://www.behindthename.com/pnl/150423