Firstly, why limit it to Irish or traditional songs?

Personally, I have no problems with women singing songs written from a man's point of view or vice versa. Would you have a problem with a storyteller telling a story from 'the other' point of view? Or an author writing about characters of the opposite sex?

Obviously some songs can have the sex changed without too much trouble (I have heard versions of 'the snows they melt the soonest' sung both ways, for instance) but for those songs that can't be changed, at least without doing serious damage to the song, then I'd say go ahead and sing it as is. Seems a bit demeaning to suggest the audience couldn't make the jump along with the singer, at least to me.