The Israeli navy intercepted two boats bound for Gaza on Friday. Israel has maintained its naval blockade since Hamas militants took control of the territory in 2007.
Of the 27 people on board, 19 remained in Israeli custody Thursday. Among them are an Irish member of the European Parliament and several other Irish citizens.
The Irish Foreign Ministry says Foreign Minister Eamon Gilmore met with the Israeli ambassador Wednesday to demand the "earliest possible departure" for the activists.
Israeli officials said Thursday that the activists refused to sign waivers that would have allowed them to leave sooner, and they are being deported on available flights.