Apart from the odd word here and there, the vocabularies of the two living branches of the Celtic language family, Brythonic (British) and Goidelic (Gaelic), appear to bear little resemblance to each other. So far I’ve only found two words that are exactly the same in Welsh, Irish and Scottish Gaelic: blas (taste/flavour) and glas (blue/green).
To some extent, differences in spelling disguise connections between the languages, but even taking those differences into account, only about 2% of the words appear to be related.
Here are a few other related words I’ve found:
Note: cú actually means hound in Irish – dog is madra. Sea is also farraige is Irish, fairge is Scottish Gaelic, and faarkey in Manx.
To find more connections between the Celtic languages, you need to go back to their earlier forms. For example, the word for true in Welsh is gwir, in Irish it’s fíor, and in Scottish Gaelic it’s fìor. These words are all thought to originate from the reconstructed form, wir or weri. Then at some point the initial w become gw in Welsh and f in the Gaelic languages.
A good place to find connections between the Celtic languages is McBain’s Etymological Dictionary of the Gaelic Language.