According to a report in The Irish Examiner, 8th Aug, an agreement exists at governmental level whereby the RAF will "conduct armed operations in Irish sovereign or Irish-controlled airspace in the event of a real time or envisaged threat of a terrorist-related attack from the skies on either this country or a neighbouring state."
RAF tornado jets could shoot down hijacked planes in Irish airspace in event of terror attack | Irish Examiner
Seems the Irish Air Corps won't be getting a capability upgrade any time soon if the service provision has been 'outsourced'.
JAS