A catchment of 300k is not big. Take Southend Airport, it has a suggested immediate catchment of 600k and has the London market on its door step, it still has a marginally busy airport. So given that comparison, Shannon probably punches above its weight. Citing that example as I know the numbers.
For airlines, big cities like Manchester, London , Birmingham are v attractive as huge catchments locally. Equally but importantly, costs like are much lower due to in many cases scale of existing business in their respective bases... Equally Shannon as an example has Cork and Kerry on one side, knock on another and Dublin too.. So competition is intense.