Of the two strains of the Celtic languages, Welsh, Cornish and Breton are connected together in the Brythonic group and have more in common with each other than the other group, the Goidelic languages of Scottish and Irish Gaelic and Manx.

http://en.wikipedia.org/wiki/Brythonic_languages

All the most common Welsh family names (there is no real equivalent Clan system) have tartans and samples can be seen at http://www.welsh-tartan.com/tartan-finder.php



Geographically speaking, although quite a distance by land from each other, they are pretty close neighbours across the Bristol Channel.