I think the realistic theory in the Taoiseachs office would be a sort of devolved region.
Some degree of self governance, like the federal system States in the US, as long as the laws of the land and taxation were aligned. And agriculture, definitely.
Mind you, I'm not sure how much they could do differently in practice. We have already seen with Brexit how difficult it is to have two systems, hence the Irish Sea border solution.
But I think the idea is to give as much autonomy as it's practically possible for a province in a small island to have. There is a recognition that many MPs in NI would only agree to it as long as they could keep Stormont and not have to come down to Dublin to the Dail. The indignity!
I suspect in practice Stormont would essentially become a very big County Council.