Belfast has two airports. I've only flown into Belfast City.
But I suspect the majority of flights are to other parts of the UK/CTA like Isle of Man and Jersey - and the EU. In fact I don't suspect, I just looked at the current flight arrivals/departures and none are from outside the EU at either airport.
So the problem is what?
Bit confused by this. The problem is Belfast International Airport. Which is an international airport (clue in name). It has flights to the UK, other EU countries, the US, Iceland and I'm sure plenty of other places too.
It is very important for the Northern Irish economy and an international point of arrival into the UK.
Indeed, pretty sure Belfast City has also had the odd package flight to Spain over the years, so it's not solely UK/CTA either.