Sinn Fein president Gerry Adams today called for the release of anti-war campaigner Mairead Maguire from Israeli custody.
The Peace People founder, 65, and former Irish soldier Derek Graham, 40, sailed on a ferry from Cyprus in a bid to break the blockade on the Gaza Strip with a consignment of aid.
Their vessel, The Spirit of Humanity, was stopped 24 miles from shore.
Mr Adams said: "The purpose of their mission is very clear. They were intent on bringing much needed humanitarian supplies to the people of Gaza. There is a humanitarian crisis in Gaza.
"The Israeli government need to open the crossings immediately and allow the free flow of aid."
Ms Maguire was the aunt of the three Maguire children who were hit by a getaway car after its driver was shot by a soldier. The deaths prompted a series of marches throughout Northern Ireland and further afield demanding an end to the violence in 1976.
Irish Justice Minister Micheal Martin has also called for the freeing of the detained pair by Israel.
They are being held in prison in Tel Aviv along with former US Congresswoman Cynthia McKinney.
Mr Adams added: "I call on the Irish government and on other EU states to demand the release of Irish and other citizens illegally arrested in international waters, that the confiscated humanitarian aid on board, which had been cleared by the customs of another EU country, Cyprus, be returned and that the boat be released and allowed proceed on its way."
Mr Martin has repeated calls for border crossings and trade links to be reopened. They were effectively shut down by Israeli authorities after the military offensive in Gaza in December and January.