When I arrived by ferry from Belfast (Northern Ireland, UK) to Birkenhead (Merseyside, England, UK), police were checking all (foot) passengers for their passports (perhaps car passengers too — I couldn't see). When asked, I was told this was because of the seamless border between the Republic of Ireland and Northern Ireland. There were no such checks when we travelled in the opposite direction.
I found those checks surprising, as this is a domestic ferry and even on ferries between the Republic of Ireland and Great Britain there should be no passport checks.
Is it normal that there are passport checks when travelling from Northern Ireland to Great Britain?