These dictionairies are worth a separate posting - I recommend MacBain's especially.
The following online searchable dictionairies on Gaelic are found at
MacBain's : An Etymological Dictionary of the Gaelic Language (Scottish Gaelic).
MacFarlane's : The School Gaelic Dictionary (Scottish Gaelic).
Kelly's : Fockleyr Gaelg - Baarle (Manx Gaelic).
Gramadach : Gramadach Lexicon. (Irish Gaelic).
Especially MacBain's is to be recommended because of its frequent - justified - citation to Latvian, Lithuanian and Old Prussian comparables. Here is a linguist who apparently knows a bit about what he is doing in citing liberally to the archaic and useful Baltic terms.