P&O Ferries says many of its services are cancelled this weekend, after one of its ships was detained in Northern Ireland for failing safety checks.
It comes amid growing anger after 800 workers were sacked over a week ago and replaced with cheaper labour.
The UK Coastguard says the ship 'European Causeway' failed tests on crew familiarisation, and crew training, during an inspection in Larne.
SIPTU's Jim McVeigh says its members are staging a rally at Dublin Port at noon today in support of the sacked workers.
"The reason why we're doing that is obviously to show our solidarity with those 800 workers. They were so brutally sacked recently in the North and across Britain. So our members and in SIPTU, in P&O, and across the port and across the city, as well as other trade unions wanted to gather to shore our solidarity."
