There is actually a long history of the use of "mercenaries" going back many centuries. While many like to characterise all such mercenaries as borderline criminal, soldiers of fortune types but that is not the same as we are talking about when I suggest the likes of Drake International (or similar) offering a interceptor solution. In fact, one could argue that likes of the Swiss Guard are technically mercenaries. Same goes for the hundreds of thousands (if not more) of contractors offering all sort of logistics and other support, often in the field.
In the case of the air domain, we have the likes of Omega Air offering aerial refuelling services as well as numerous organisations offering training up to and including aggressor services. Others offer surveillance services.
So before making comments about "Outsourcing a sovereign democratic government function to for hire mercenaries" please remember that a lot already happens and indeed has been the case for many years.
In the case of the interceptor role here, I see this as likely being more akin to an air policing style role. After all, it's not like Ireland is facing a frontline combat threat of bombers or the likes crossing their borders. More than likely the real threat here is likely to be wayward airliners or other commercial aircraft, possible smugglers, or the like.
As to the cost effectiveness question, yes private contractors are looking for a profit. there is nothing wrong with that. ?However the advantage they offer, and where it often does become attractive for governments is that by procuring a service the government doesn't need to pay for acquisition, upgrade and maintenance costs not to mention the broader military organisation. Yes, often similar are rolled up into the contractor price but the contractors have the ability to amortise that across multiple customers thus reducing the price for individual ones.
