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:

Gaeilge (Irish)	Gàidhlig (Gaelic)	Gaelg (Manx)	Cymraeg (Welsh)	Kernewek (Cornish)	Brezhoneg (Breton)	English
capall	(each)	cabbyl	ceffyl	(margh)	(marc’h)	horse
gabhar	gobhar	goayr	gafr	gaver	gavr	goat
cath	cat	kayt	cat	kath	kazh	cat
cú	cù	coo	cu	ki	ki	dog
abhainn	abhainn	awin	afon	avon	(stêr)	river
muir	muir	mooir	môr	mor	mor	sea
agus	agus	as	a/ac/ag	hag	ha/hag	and
aimsir	aimsir	emshir	amser		amzer	time
ainm	ainm	ennym	enw	hanow	anv	name
airgead	airgead	argid	arian	arghans	arc’hant	silver
anáil	anail	ennal	anadl	anal	anal	breath

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.