I thought it was already pretty well understood that "Celtic" is only meaningful as a linguistic grouping, but it seems the old idea of a separate "Celtic race" or "Irish race" is pretty strongly embedded, even now:
This makes me think about wider issues. I don't know how many online discussions I've been in recently in which I've been solemnly assured that humanity is divided into three races. (Three shall be the number thou shalt count, and the number of the counting shall be three. Four shalt thou not count, neither count thou two, excepting that thou then proceed to three. Five is right out.) And people will go on believing this, even when genetic evidence makes it perfectly plain that there's no such thing as race, never has been and never will be. There are heritable phenotypes, some of which are clustered together as a result of geographical or historical accident, none of which are set in stone and almost all of which are continuous rather than discrete states. The weight we assign them is entirely cultural.
As always, Darwin puts it elegantly: "Man has been studied more carefully than any other animal, and yet there is the greatest possible diversity amongst capable judges whether he should be classed as a single species or race, or as two (Virey), as three (Jacquinot), as four (Kant), five (Blumenbach), six (Buffon), seven (Hunter), eight (Agassiz), eleven (Pickering), fifteen (Bory St. Vincent), sixteen (Desmoulins), twenty-two (Morton), sixty (Crawfurd), or as sixty-three, according to Burke. This diversity of judgment does not prove that the races ought not to be ranked as species, but it shews that they graduate into each other, and that it is hardly possible to discover clear distinctive characters between them."