The demonstrators blockaded the port for two hours as nothing got in or out, with a 61-year-old haulier charged with road traffic offences after trying to force his way in.
RMT general secretary Mick Lynch said: "The protest at Cairnryan displayed the full frustration and sense of injustice felt not only by sacked P&O Ferries workers but also that of the wider community.
"Until all P&O ships are impounded and brought under government control, protests and demonstrations will continue."