The prime issue seems to be landing rights, and third country agreements negotiated by the EU as a whole on the part of its members. Overflying seems, despite what some of the red tops say, to be a non issue. This could be ironed out by some sort of a transitional arrangement, some legal version of "business as usual" wrapped up appropriately in legalese.
The problem with all of this, is that the EU negotiators have described this sort of approach as "Cherry picking", which they have specifically ruled out. There are many other technical issues across most industries, each of which consider their baby to be critical for future operations. The problem is, until the conservative party have a clear, consistent, and defensible approach to these negotiations, nobody is going to get around to the details, and it is not going to happen.
For the Irish, it is a big deal, and so their prime minister is going to talk about it.