I was under the impression that (most of the) the international squad (unlike say, Tommy Bowe, who seems to like a career with a mediocre team like the Ospreys) is contracted to the IRFU and “released” to the provinces (Leinster, Connacht, Ulster, Munster) for Heineken Cup, Pro 12 (or whatever it’s called this week), etc.
So when the 6N is on, the players are called back centrally and only released if not needed in the international squad. I have no idea what the salary conditions are though.
I had a quick google and couldn’t turn anything up. If you look on the Beeb though, you’ll see a recent story where PO’C signed a deal with Ireland & Munster, not just with Munster. I don’t really know how the intricacies work.
The system has probably contributed a lot to the resurgence of Ireland in the 6N from (very) occasional successes to a bit more consistency because you don’t get your fly half or no 12 getting injured on a weekend when they’re not needed for international duty.