Is this relative form of the verb very common? I've never seen it in books (not yet...)
Here's what I mean:
an fear a ghlana(nn)s an carr = the man that cleans the car.(present tense)
an fear a cheannaío(nn)s an carr = the man that buys the car. (present tense)
an fear a ghlanfas an carr = the man who will clean the car. (future)
an fear a cheannós an carr = the man who will buy the car. (future)

It is used, I think it is common since it can be found not only in books, but it also belongs to the vernacular. BTW de Bhaldraithe uses it in his English-Irish dictionary, so...

Le meas, Peter

The relative form is common in Connemara and Ulster, but seems almost unknown in Munster, and unknown in the Standard as well (except in two or three phrases like "a leanas"). And because of the Standard, you won't see it much in books. But it is really very common in speech in Connemara and in Ulster dialects, and in the books that were written in these dialects (when they have not been modified by the editor!).

Exactly - recently I received "caighdeanised" version of Seadna. What a shame what they do the original text! Changing fé to faoi can be somehow justified by substituting "rinne mé" for "do dheineas" is a step too far in my opinion!