Who says they're "celtic" words? My understanding is that these counting words run approximately from Northumbria to Gloucestershire and are also known to some extent in the Welsh border country. They appear to be unheard of in Ireland and Scotland.
The only authoritative source I have read suggests that they may be pre-celtic and belong to an earlier wave of population but according to their spread they might just as easily be "Anglo Saxon" or even Norman French (though there is no etymological argument for either of these, just as there is none to associate them with any "celtic" language).
In parts of the Danelaw, particularly Yorkshire, there is a counting rhyme which goes like this (I'm sure I've missed a few).
Eeny, meeny, mackeracker, ee, aye, dominacker, ... chickeracker, om pom push.
Does anybody know any better versions of this?