Another common popular opinion penalty is the "offensive war" modifier.
As the name implies it ticks up when you are in an offensive war (that is one you declared) and ticks down while you are at peace (I think it stays static if you are only in defensive wars).
This modifier is, in my experience, often pretty relevant for foreign culture group/religion provinces since it's easier for the war penalty to push those into rebellion because of their naturally lower base opinion.
Specifically the penalty starts after 6 month in offensive wars and once it starts it increases by -1 every other month.
Once the penalty is there it takes 6 months at peace before it starts ticking down, which also happens by 1 every other month.
Note it doesn't have to be the same war, going to war with a bunch of welsh counts one after the other increases it just as much as one big war if the first takes enough time to give you a penalty and you don't take a 6 month break in-between.
If this is happening to you, you can avoid the rebellion by simply outlasting it, after a few years at peace the penalty will go away and the rebellions will stop.
In the future you can prevent new rebellions by either only declaring short wars (less than 6 months) or by taking a break between each war of conquest (I find that a year or so generally keeps everything nice and cozy.)
Finally it's worth noting that it's the culture and religion of the holder of the county that matters, not the top liege, so you could give the land to a welsh courtier and keep your domain in Irish land instead if a vassal with opinion penalties is easier for you to deal with. (say by swaying or putting on the council or just having them be happy from being granted the title until there has been time for cultural tolerance to build up from sharing a realm)
