Presumably to connect with their own country ferries from NI would to arrive in Liverpool as opposed to Stranraer or Troon (or is there any ports further north in England than that, could Barrow play this role?)
Barrow is the biggest port between Liverpool and Clydeside, though there are also small ports along the West Cumberland coast (e.g. Whitehaven) that could presumably be used as ferry ports if there was suddenly demand. Would assume that at first you'd just see a huge increase in ferries to Heysham (which already has a link to Belfast) though. Or maybe finally prosperity will return to Fleetwood!