No, it’s singular after bigger numbers (like twenty, thirty, hundred, etc.).
It used to be genitive plural, because those bigger numbers worked as nouns, a bit like a score in English. So eg. fichead fear meant a score of men, twenty of men where fear was the gen.pl. of men. But since many common masculine nouns (like fear) have genitive plural identical to nominative singular, the usage of singular became common in general.
In more traditional conservative Gaelic twenty dogs would be fichead chon, since cù has irregular gen.pl chon, but that’s not modern usage (and Google finds this phrase only once in an issue of The Celtic Review from early 20th c.).