There's a route from Dublin to Belfast (somehow) which the Maracycle charity ride uses
I don't know if it's nice for touring, but I can ask the CTC boys and girls.
In theory a group ride meeting at Newry (el train) , then all tiddle up to Belfast on bikes, and then go up north for some lovely scenery and campsites.
Unless there's blizzards. Then it's bikes in train to Belfast and a pint with some pies and irish stew. Yum