If you have a drywall substrate why not finish it off with hot mud (aka setting type compound) instead of plaster? You can mix it to whatever consistency you want and work it like plaster. You can, with experimentation add texture. I have even squeezed it through cake decorating tips to satisfy interior designers a time or two.
I am not sure what you hope to gain with actual plaster unless you are going to try to lime color in it or going for a polished Venetian plaster look or something. If either is your goal, I guess I would lay down a coat GARDZ first. Otherwise, just a quality sealer/primer should do.
I repaired plaster with plaster all the time but once I had drywall up I seldom saw a reason to coat it with plaster. If I were doing a Venetian look I would still use hot mud and cover it with artist's gesso or modeling paste. Something with marble dust in it. Wasted if you are just going to prime and paint it of course. Better to use a specialty paint with some metal or pearl in it I think?
Nothing against plaster by the way. And if you are trying to maintain that Burly-Manly-Man-Ring-of-Kerry look of yours, you should use a hawk and plaster because it weighs twice what compound does and those big biceps will attract psychotic red-headed women faster!
Not sure about Ireland but hot mud comes in a range of cure times ranging from something like 5 minutes to 120 minutes (hard to find the 120). 5 minutes starts the moment it sniffs a hint of moisture but is great if you can work fast and have somebody mixing for you. You should be able to get 5-90 at a real paint store. Not sure what the box stores carry since I do not shop in them.