normal route for a trade like this is to labour for a plasterer for a while-how long it takes all depends on who youre with, and you learn from this. Normally you'd be in front of the mixer with a shovel and feed the mix to the plasterer. Depending on how "hungry" the lad your labouring for and if youre doing insides or outsides or dashing, it can be seriously hard work.
you can contact FAS for info on apprenticeships. you might find an adult evening class in your nearest Tech college.