Darren Proctor, from the RMT union, said workers are ramping up their protest at both Larne Port and Cairnryan.
Former P&O workers have blocked an entrance to Larne Port in a ramping up of protest action.
Protests have been ongoing at the port since P&O’s decision in March to sack almost 800 workers without notice and replace them with cheaper agency staff.
At around 2pm on Friday, protesters took part in a blockade at an entrance to Larne Port. The action lasted around an hour-and-a-half.
Similar action was taken at Cairnryan Port in southern Scotland.
It comes after an incident on Tuesday in which the P&O vessel European Causeway lost power in the Irish Sea.
The ferry was adrift five miles off the coast of Larne for more than an hour.
Darren Procter, from the RMT union, said workers are ramping up protests over safety concerns.
He told the PA news agency: “With the events of this week and the European Causeway, it’s something that we as a union have identified is going to be a problem, and it was a problem this week so we decided to have a blockade in Cairnryan and also in Larne.
“This is something we’ll be looking to do more and more because we don’t believe those vessels are safe, we’re not happy with what P&O have done to the workforce and the impact that it has had. Just because the individuals have been sacked and have received the money, it is not the end of the issue.”
A Police Service of Northern Ireland spokesperson said around 40 people took part in the action.
“Police received a report of a small protest at approximately 2pm today, Friday April 29, at Larne Harbour,” they said.
“Approximately 40 people were in attendance.
“The protest ended and no issues were reported.”