The population of Limerick city is roughly 60,000 right? That's about half the size of Swords....
Shannon has a tiny catchment area in the grand scheme of things. It punches way, WAY above it's weight already in terms of Trans Atlantic traffic. It has more daily flights than GLA, EDI, BHX, NCL, BFS and countless far larger cities in Europe. But it just can't match those same cities for flights to other European cities due to not having the catchment, most of those TATL flights are kept running by inbound US pax, we just don't have the same draw in most of Europe because that's not where the diaspora went.
Even 10 years ago, how long and inconvenient a journey would it be to see Belfast, Dublin, Kilkenny, Cork, Killarney,The Cliffs of Moher, Galway etc? Back then it made sense for tourists to fly in to one airport and out from another, now with the new road network on the island that is no longer a requirement, hence no more Lufthansa, Air France etc in Shannon. It's depressing I know but it's the way of the aviation world right now, in this case consolidating at hubs, then increasing frequencies