There are other loopholes. Last night I was in the company of an Irish couple who sailed across on Monday. On board were 5 men speaking in a language nobody could identify. They had obvious difficulty when asking for refreshments although one of them finally recognised 'coffee'.
The husband uses the ferry on a very regular basis and when talking to a ferry staff member of his acquaintance, he mentioned the 5 men. He was told they could be part of a very regular traffic of illegal immigrants who would fly into Dublin then the nip across the border to Belfast for the boat to Stranraer. There are no customs checks when anyone arrives on Scottish shores and the police have long since given up doing the work of the UK Border Agency. The immigrants quickly disperse - usually south of the border.
Supposedly, the Irish immigration service should check incomers. That is how the EU with its common external border is supposed to work.