From: Raggytash
Date: 30 Dec 17 - 04:53 AM
"Even crossing the Irish Sea random checks occur and passports frequently checked"
Really ! ? ! ?
1. As a frequent traveller to Ireland for more than 20 years I have seen two vehicles stopped and searched, they were both inward bound to the UK and searched in Holyhead by UK Border force.
2. UK Nationals do not need a passport to visit Ireland (I think the same is true of Irish Nationals visiting the UK).

Just because you don't see the checks doesn't mean that they don't happen.
Q: When would be the best time to check vehicles for stowaways (using infra red) or for explosives/drugs (using sniffer dogs etc.)?
A: When the vehicles are unattended on the vehicle deck, and all (legitimate) travellers have been sent to the upper decks.