Why is it believed that characters from the Irish myth cycles were once gods?

Question

I have a passing familiarity with some of the stories from Irish myth, at least in their translated form. One thing that has always puzzled me about these tales is the presumption among scholars that the characters represent gods. Yet the stories themselves almost exclusively talk about these characters as though they were heroic mortals.

This is stated explicitly in Wikipedia, rather more succinctly than I can manage but without references:

Texts such as Lebor Gabála Érenn and Cath Maige Tuireadh present them as kings and heroes of the distant past, complete with death-tales. However, there is considerable evidence, both in the texts and from the wider Celtic world, that they were once considered deities.

And

While we may suspect a few characters, such as Medb or Cú Roí, of once being deities, and Cú Chulainn in particular displays superhuman prowess, the characters are mortal and associated with a specific time and place

Since these stories were being recorded by Christian monks, it is certainly plausible that they changed the representation of the characters into historical people rather than gods so as not to affront their religion. But what actual evidence is there to suggest these characters were once iron age deities?

Answer 1

There are several arguments in favour of the characters in Irish and Welsh myths being deities rather than heroic mortals. One is etymology: if the Irish champion Ogma has a name similar to that of the Gaulish god Ogmios, for example, then we can assume that he may also have been a god. This is true for many Irish and Welsh "deities", including Danu, Brigit, Rhiannon, and Lugh.
Another is that the Norse myths are told the same way: Snorri Sturluson and Saxo Grammaticus were both Christians, who described Odin and Co as outstanding mortals, not gods.
Third, many of the stories told of the Irish and Welsh deities take the pattern of other stories about other deities. (The suffering horse-goddess theme, for example, takes in the Welsh Rhiannon, Greek Demeter and Medusa, and Indian Saranyu.)

It is a good question, though, because occasionally someone does still argue that the Irish Tuatha de Dannan or the characters of the Welsh Mabinogion are not deiites, and they do have a point, in that the texts don't describe their characters as divine figures. So writers are making an assumption when they describe figures in Irish and Welsh myth as deities, but I think a justified one.