hey! I'm a bit stuck. I'm trying to work out why sometimes there is no genitive plural given for nouns in O'Donaill's dictionary?
I assume every noun has a genitive plural unless it's a word that doesn't have a plural?
So is the reason something to do with strong plurals? Would O'Donaill be assuming that you know from the nom. plural if it's strong and therefore the gen. plural is the same as the nom. plural given?

The genitive plural is only stated separately in Ó Dónaill if it is different from the nominative plural. If the genitive plural reverts to the nominal singular form you will see "gpl. ~" Otherwise just assume that genitive plural is the same as the nominative plural (if there is one).