An Irish and a Canadian aid ship are currently in international waters on their way to Gaza to take part in a humanitarian mission.
In July, the Irish ship, the MV Saoirse, was at the centre of allegations it had been sabotaged by Israel.
A chunk was found to be missing from the ship's propeller shaft which activists claimed could not have been done accidentally.
Socialist MEP Paul Murphy, former Fianna Fáil TD Chris Andrews and two councillors from Sinn Fein are on board.
Israel will not permit the flotilla of pro-Palestinian activists who set sail from Turkey to reach their destination, a government official has told Reuters.
"They will not be allowed to breach the blockade," the official said, speaking on condition of anonymity.
"The blockade is on, and no exceptions will be made," he said, adding that Israel "will take whatever measures will be necessary."