Well, not really. Even in English there are nouns like 'fish' or 'sheep' that are take the singular in a plural situation. (Horse used to be another such word, but I think that would be considered archaic by now.)
Maybe you could see it as a bunch situation - you can have many grapes in a bunch, but the bunch is singular. When it comes to number in Irish it seems like a lot of words turn into bunches. (I asked this question earlier and the penny just dropped.)
Hope that analogy helps a little.
"fish" and "sheep" are admittedly slightly different than nouns in Irish, as "fish" and "sheep" use the same singular and plural form, whereas "madra" and "madraí" are both rather different.
That aside, I quite like your bunch analogy, and I suspect that that will help me quite a bit. Thanks for the assistance.