The immediate catchment of Shannon is the entire counties of Clare and Limerick, which have populations of 111,000 and 184,000 respectively.
However, as none of Cork, Kerry, Galway or Knock airports have transatlantic services, Shannon becomes the nearest transatlantic airport for people living in Cork (481,000), Kerry (140,000), Tipperary (149,000), Galway (231,000), Roscommon (59,000) and Mayo (124,000) as well as being an option for people living in counties like Offaly (71,000) and Westmeath (79,000).
Combined, that's almost 40% of the population of Ireland. As long as Dublin remains the only transatlantic alternative, Shannon's business should remain strong.