It is usual to find Proto-Celtic reconstructions in Wiktionary that assume that PIE [ei] became [ē] in Proto-Celtic. I don’t know who came up with this (Matasović?) or why, but I must strongly disagree.
There is evidence that [ei] became [ē] in Gaulish (probably late Gaulish), an apparent instance of this being deuo– ‘god’, i.e. dēuo– from *deiwos. But I hope this isn’t the basis for assuming that [ei] had already become [ē] in Proto-Celtic. This would be like saying that Latin must have had the [ø] sound because French has it, which would obviously be absurd.
It may also be that [ei] became [ē] at some stage in Proto-Goidelic. This [ē] would have shifted to [ea] and then to [ia] as is seen in Old Irish (written ía): e.g. OIr íasc ‘fish’ from *eiskos (< *φeiskos < *peiskos). But note that the [ē] (written é) remained in contexts where i-affection inhibited the shift: e.g. OIr éisc, genitive singular and nominative plural of íasc, both from *eiskī; also OIr léicid ‘to leave’ from *leikwī– (not *linkwī-; cf. PGmc *līhwaną < *leikw-). The [ē] in Proto-Goidelic obviously came from [ei] but this development occurred in Proto-Goidelic, not in Proto-Celtic.
The Brittonic languages give strong evidence that [ei] did not in fact become [ē] in Proto-Celtic. The reflex of PIE [ei] in Brittonic was [ui] as represented in Welsh by wy and in Breton by oue. It is quite unlikely that [ui] could have developed from [ē]. Much more likely is the following transition: Proto-Celtic [ei] > [oi] > [ui].
By the way, this resembles the development of Old French [ei] into Modern French [wa] : [ei] > [oi] > [oe] > [we] > [wa] with a shift from falling to rising during the [oe] stage (although this development occurred later than the aforementioned Brittonic development).
I might also mention just for interest’s sake that PIE [ejV] (where V stands for any vowel) became [ijV] in Proto-Celtic. Of course, this does not exclude the possibility that PIE [ei] became Proto-Celtic [ē] (but it didn’t happen).
For what it’s worth, my Vindonian language has the following development of Proto-Celtic [ei]: [ei] > [ē] > [ie] with a shift from falling to rising during the [ē] stage. The Vindonian word for “food” is bied which may be compared with Welsh bwyd and Breton boued, all of these being from Proto-Celtic *beiton (not *bēton).