Under the Air Navigation Foreign Military Aircraft Order 1952, foreign military aircraft (this has historically included Soviet planes) can fly through Irish airspace and use the airports, but only under strict regulations and with clearance from the minister of foreign affairs. This is subject to conditions including that the aircraft is unarmed and doesn't carry weapons.

So if it's carrying weapons, that's a violation of the law. But the government have been to spineless to actually inspect planes to confirm there are no weapons.