Eight Irish people were travelling on a flotilla of aid ships where 15 people were reportedly shot dead today by Israeli commandos.
Michael Martin, Foreign Affairs Minister, said consular officials were working to confirm the safety of Irish people caught up in the incident.
"I am gravely concerned at the reports emerging of the storming of a Turkish ship this morning by Israeli commandos," the minister said.
"My department is seeking to establish the full facts of what has occurred and confirm the safety of the eight Irish nationals who sailed with the Turkish-led flotilla.
"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."
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 is understood the Turkish Mavi Marmara ship, from the pro-Islamic aid group IHH, and Free Gaza's Challenger 1 were intercepted by Israeli Navy ships in international waters, 80 miles off the coast of Gaza.
Three Irish people, Dr Fintan Lane, Fiachra O Luain and Shane Dillon were on board the Challenger 1 which had travelled from Cyprus in the first wave of the flotilla.
The Ireland Palestine Solidarity Campaign (IPSC) said it has not been able to contact any of its members on the ships.
Dr David Landy, IPSC chairman, 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 Irish owned vessel, the Rachel Corrie, is part of the convoy.
Footage on the internet appeared to show a video of pandemonium on board the Mavi Marmara, with activists in orange lifejackets running around as others tried to help a colleague lying on the deck.
The Turkish website also showed an Israeli helicopter flying overhead and Israeli warships nearby.
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.
Mr O Snodaigh called on Mr Martin to expel the Israeli Ambassador to Ireland.
"It looks as though this cold and calculated attack took place in international waters, in breach of all international maritime laws. There must be action from the international community," the Dublin TD said.
He added: "At all stage of preparations to join the flotilla the organisers reiterated non-violent and peaceful resistance to any boarding by Israeli army. The Israelis had nothing to fear from this flotilla.
"My thoughts are with the families of those who died. They were courageous men and women as was each and every person on the flotilla."
Nobel peace prize winner Mairead Maguire was on board the Irish ship MV Rachel Corrie which left Cyprus yesterday in the second wave of the flotilla.
In 2009 she was arrested and detained in Israel after attempting to defy the blockade by sailing a boat load of aid into Gaza.
Ireland's Ambassador to Israel was due to attend a briefing with authorities in Jerusalem while contacts were ongoing between Irish and Israeli officials in Dublin.
Israeli media reported one Irish person had been wounded.
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 Department of Foreign Affairs said the Minister has summoned the Israeli Ambassador Dr Zion Evrony to a meeting in Dublin over the incident.
Meanwhile, Mr Andrews condemned the incident and called for the ambassador to be called to account.
"I am deeply shocked but sadly not surprised by today's events," the Dublin TD said.
"Israel has proven repeatedly that it believes itself to be above international law, evidenced by the fact that it carried out these attacks in international waters."
Mr Andrews "This attack took place in international waters. This fact cannot be disputed and therefore there must be an international response," he said.
"Nothing short of a full independent investigation led or overseen by the UN will be acceptable."