uibheacha is the plural (both nominative and genitive) in some dialects (I think most forms of Connacht). “uibhe” is the standard plural and also the special counting plural.
Note that what at least one person above said about nouns following the verbal noun (ag ithe, ag ól, etc.) is true in certain cases but not true in others. The actual rules that govern this are somewhat complicated. Generally speaking, you will see the following noun be in the genitive if it is unqualified, i.e. if it is not followed by an adjective (i.e. ag ithe úill = eating an apple, but ag ithe úll mór = eating a big apple) or if it is itself another verbal noun. At this point I would really not worry too much about the intricacies of this but I figured it was worth mentioning.