There is a process in the GE75 plan where frequencies can be reallocated (or parameters modified) by mutual consent between all parties with allocations on the same (and possibly adjacent channels) which is how Ireland got the 549 frequency in the first place.

Ireland has quite a few other allocations (unused) which could be used at Monaghan.

Can't see radio Caroline using it without the blessing of the Irish authorities since OFCOM would take a dim view of this and could revoke their UK licences (there is precedent for this).