Two ferry services run from Cairnryan to Northern Ireland – Stena Line connects with Belfast and P&O with Larne (around 30 minutes north of Belfast). Each service operate 2 ships which cross midway. Both services operate similar schedules throughout the day, with slightly staggered times.
Both Stena & P&O show updates of sailings on their websites, but they are not always up to date. Two products (Shipais and Marinetraffic) allow access to ship updates to monitor progress – links are shown here).