dinneen's dictionary has "cé ra díobh thu?" implying that cér could actually be "cé ra".
Cér a mac thú? - what page is this on in the old Christian Brothers' Grammar. I can't find it. Do you mean the old Green edition produced around 1900? Cér a mac thú? must be the same as the "cé ra" in Dinneen's. It is not a possessive "a", and is not a relative particle (and so not a shortened form of "go").
The trouble is you can read hundreds of pages of Irish before coming across one example of something like this. I might need to do a lot of reading to find any more examples.
Oh! I just saw! At
http://wikisource.org/wiki/Page:Seadna.djvu/212, there is an example of cér díobh í in the non-proofread version of Séadna, but the image on the right shows the original to be "cé 'ro díobh í". It seems there is a vowel there, in Cork Irish at least.