There are numerous flights that operate with so-called 'fifth freedoms', and there are even treaties of 'cabotage' allowing foreign airlines to operate domestic flights within a different country - although I can't think of any flights ex UK that take advantage of that - EU Open Skies is a different step altogether.
Prior to Deregulation Portugal, Greece and Ireland (Ironically) had many fifth freedom routes in place, e.g. I can't think of that many examples:
Dublin/Manchester/ many points in Europe with AerLingus
Dublin/Bristol/Brussels Aer Lingus
Dublin/Birmingham/Copenhagen with SAS
Dublin/Manchester/Lisbon with TAP
An example of cabotage: LHR-BFS on EI
With a deregulated EU air market, there is no need for any such special arrangements (or Laws) or 'fifth freedoms' due to deregulation?? Is there?
Describing Aer Lingus ex Belfast as a Cabotage is technically fitting the definiton but many in NI or on the Island of Ireland would not see Aer Lingus as a foreign airline!!
EI-BUD