I find it rather unusual that Dublin will have 29 (11 Air Canada, 7 Aer Lingus, 7 West Jet, 3 Air Transat, 1 Europe Airporst) weekly flights to Canada during the summer, yet the rest of Ireland (NI Inc) does not have one.
Easy one. Dublin and its surrounding area is by far the largest conurbation in Ireland containing over a third of the Republic's population. It is also the richest part of Ireland and the seat of government in what is a very centralised country. Cork, Limerick and Galway pale into insignificance in comparison, (and since you include the North, so does Belfast).