"Roisin Dubh", though clearly the words are those of a man addressed to a woman, I've only ever heard sung by women except one recording by Frank Patterson; Burns's "Banks of Doon", though it gains some additional significance/ambiguity when it's recognised that it's made from a woman's perspective, is regularly given by male singers; more generally, I've often heard female Irish traditional singers perform songs which, logically, are in the voice of a man, and - not quite so frequently - male Irish trad singers perform "female" material. Do what seems right; and some comic songs allow plenty of opportunity to vary your voice (whatever "purists" might say), such as "Mrs McGrath" and "Phil the Fluter". I've always liked "Biddy Mulligan"; but would only sing it as a joke, and, yes, in Falsetto.