We usually do the Cairnryan - Belfast (Stena) route, but are considering Liverpool overnight this time. Are the Liverpool ferries as hideous as everyone says now they've been upgraded? We love the Cairnryan crossing, but live at the opposite end of the country, so it's a huge drive.
Other alternative is Fishguard, but I'm assuming it's a pretty long drive to Belfast (& on up to the north coast)