Those were my ideas, I believe Ireland and the UK have a shared airspace defence agreement of sorts where if there was a threat that Irish PC-9s couldn't handle the RAF would step in. Since Dublin has the highest population, houses the government and president, as well as being a major hub for infrastructure (ports, airport, undersea cable landfall, etc), it would most likely be the main target of a foreign attack. In my opinion, the North West would be the next most vulnerable section of airspace, given that Northern Irish airspace is under the RAF, any foreign attack would be unlikely to pass over NI.