There are matters that have been mentioned, but missed.
1. Ryanair aircraft are registered in the ROI. However the ROI is covered by the European Convention on Human Rights, and is in Irish law.
2. Mention has been made of the various acts in the UK that cover being airside. None of these have powers of arrest attatched to them which include 'any person', nor are they arrestable offences under the Police and criminal evidence act. Therefore a police officer may be able to arrest, an aircraft captain cannot.
Speaking as a non pilot with no axe to grind, it sounds like Ryanair trying to avoid extra delay at a divertion, and has nothing whatsoever to do with security.
As to contractual reasons for a detention, that would be like saying if you go to a restraunt, you will not be permitted to leave until you have eaten the meal you ordered. While I understand air transport is not exactly the same, the principle is.
NigelOnDraft is almost right I would say, except it is for the airport managments security to provide an escort etc if there is no handling agent. A pax in these circumstances has not committed an offence until he actually leaves, and even then it is debatable.
As for PC Plod having a cup of tea with the crew, mine was always coffee with 2 sugars!