Taoiseach Brian Cowen today blamed the Israeli blockade of Gaza for a commando assault on a flotilla of aid ships which left at least 15 people dead and scores injured.
Mr Cowen accused Israel of breaking international law by banning humanitarian shipments and called for an international inquiry into the naval operation and killings.
Three Irish campaigners - Dr Fintan Lane, Fiachra O Luain and Shane Dillon - were on board the vessel Challenger 1 which had travelled from Cyprus in the first wave of the flotilla and was reportedly boarded by Israeli forces.
It is believed one of them was injured. His condition is not known.
Mr Cowen said: "The reason this has happened is there's a blockade taking place at the moment in respect of humanitarian assistance being provided to Gaza, to the people of Gaza.
"I believe that is in violation of international law. People are entitled to have humanitarian assistance."
Michael Martin, Irish Foreign Affairs Minister, summoned the Israeli Ambassador Dr Zion Evrony to a meeting in central Dublin this evening.
"The reports of up to 15 people killed and 50 injured, if confirmed, would constitute a totally unacceptable response by the Israeli military to what was a humanitarian mission attempting to deliver much needed supplies to the people of Gaza," Mr Martin said.
The Turkish Mavi Marmara ship, from the pro-Islamic aid group IHH, and Free Gaza's Challenger 1 are understood to have been boarded by Israeli Navy ships in international waters, 80 miles off the coast of Gaza.
Internet footage has emerged showing pandemonium on board the Mavi Marmara, with activists in orange lifejackets running around as others tried to help a colleague lying on the deck.
Israeli authorities accused the crew of one of the ships attempting to stop a boarding party, attacking commandos with handguns, knives and metal bars.
The flotilla, which had been warned it would not be allowed to pass an Israeli sea blockade, was carrying about 10,000 tonnes of aid and about 800 passengers on eight ships. It left Cyprus yesterday despite warnings.
Nobel peace prize winner Mairead Maguire was on board the Irish ship MV Rachel Corrie in the convoy. In 2009 she was arrested and detained in Israel after attempting to defy the blockade by sailing a boat load of aid into Gaza.
Mr Cowen questioned whether the Israeli Navy had the right to board ships in international waters.
"Whilst the Israel Government have withdrawn from Gaza they retain a de facto occupying force since they decide what gets into Gaza and who gets of it," the Taoiseach said.
"I believe therefore that the cause of this problem relates directly to the fact that there is a humanitarian blockade taking place against the people of Gaza.
"I question whether in fact there could be a legal basis for Israeli forces coming on to ships in international waters. This was a humanitarian mission."
The Ireland Palestine Solidarity Campaign (IPSC) said it has received reports from some Irish people on the second wave of the flotilla but has been unable to make contact with the three who were on board Challenger 1.
IPSC chairman Dr David Landy accused Israel of breaching international law.
"The fact that Israel would allow its forces to kill and wound international human rights activists shows the world once again that Israeli is a rogue state that acts with impunity," he claimed.
The Department of Foreign Affairs said staff were trying to contact Irish citizens on the flotilla directly and also through the campaigners' co-ordination centre in Cyprus.
It is understood boats involved in the flotilla will be escorted to the northern Israeli port of Haifa.
The IPSC has planned a demonstration in Dublin tonight from O'Connell Street to the Israeli embassy in Ballsbridge.
Speakers at the protest will include Sinn Fein TD Aengus O Snodaigh and Chris Andrews, Fianna Fail TD, who were refused access to the flotilla by the Cypriot authorities and had to abandon plans to travel with aid workers.
The Oireachtas Committee on Foreign Affairs also announced plans to summon Dr Evrony to an emergency meeting later this week.