I don't know much about D&T's specific setup, so this is a very generic answer. Hopefully it will still help though.Playing with Death and Taxes mod, I Scotland, control all of Ireland, not core. I can't release Ireland yet I can release Wales?
You need at least one province with a culture in the same group as the primary culture of the country you're looking to release. Then it will release with all its cores, even the ones that have the wrong culture.
I think the problem is likely that there are no provinces with Irish culture (or one in the same group) to release it from, or there never was an Ireland nation, so they don't have cores.
If the first is true, there is unfortunately no real way around it, aside from selling a province to another nation within Irelands group and hoping they convert its culture. If they do, you can conquer it back and then finally release it. That is assuming there is even another nation in their group that would buy that province, which is unlikely.
If the second is true, can you release anything at all in Ireland? If you can, you can just sell the other provinces to the nation you can release. They won't be able to form Ireland while they're a subject, so if that was the goal, I'm afraid I can't help with that.
disclaimer: It could be that D&T works a little bit different, so if this doesn't help figure it out, you should probably ask again on their user mods subforum.