No. It's partly idiomatic, partly necessitated by the narrow meaning of 'tha + aig.' It's idiomatic because literally "I will be at fishing" (or whatever) is a way of saying that one does something habitually. Not just now, but I WILL be at fishing in the future (because I do it often). And it's partly necessitated because 'tha mi ag iasgach', the present tense, only refers to an action occuring now. Literally 'I am at fishing.' It cannot represent habitual action because that implies other events than the one now. If you transliterate then it makes sense. That's a bit unclear I fear, but hope it helps a bit.