I stand corrected.
UK and Ireland opted out of the "Schengen Rules" (open borders)
Most airlines do require a valid passport.(sorry)
Photo Id is all that is required on the Dublin-Liverpool crossing,so logic would sugest that all thats required on the Larne-Troon crossing.
"A passport for all passengers (including children and infants), valid beyond the date of return, is required for all trips outside the United Kingdom. Photographic identification must be carried and shown when required for all trips between Dublin and Liverpool."