Taoiseach Micheál Martin has condemned the interception by Israel of an aid flotilla with at least 10 Irish citizens on board and called for the immediate release of those detained.
The Government is calling on the Israeli authorities to uphold their obligations under international law.
"I strongly condemn the interception of boats in the Sumud flotilla in international waters and the detention of those on board by Israeli military forces, and call for their immediate release," the Taoiseach said in a statement.
Mr Martin added that the Government is in direct contact with the Israeli authorities.
"The Government has raised concerns for the welfare of those detained with the Israeli authorities.
"Such interceptions and detentions are wholly unacceptable and must stop. The Government will also discuss with EU partners how we can ensure the safety and wellbeing of our citizens", he said.
Dr Margaret Connolly, the sister of President Catherine Connolly, is among those on board.
"How many times will Israel kidnap Irish citizens before our Government sanctions it?" her husband, Sligo councillor Declan Bree, said on RTÉ's Six One, adding that it "spoke volumes" there had been no statement from the Taoiseach or the Government.
However, the Government this evening condemned the interception of the flotilla, which was intercepted around 70 nautical miles off Cyprus.
The Global Sumud Flotilla said earlier it had lost contact with the intercepted boats.
"We condemn the interception of vessels in international waters and call on the Israeli authorities to uphold their obligations under international law to ensure the safety and welfare of all those on board the flotilla," a Government spokesperson said.
"We are aware of the ongoing Israeli operation in international waters to the west of Cyprus, and that this is impacting a number of Irish citizens.
"While the Department of Foreign Affairs and Trade (DFAT) has long advised against all travel to both Israel and Palestine, our immediate priority is the safety and wellbeing of Irish citizens on board the Flotilla," they added.
"Officials from across the DFAT, including those in our embassies in the region, continue to liaise with the relevant authorities, and stand ready to provide consular assistance to Irish citizens that need it," they added.
Watch: 'I'm very worried about her,' President Catherine Connolly says on sister's detention
The flotilla posted videos from Dr Connolly and five others earlier today, which appear to have been recorded in advance of their detention.
In her video, Dr Connolly said: "If you are watching this video, it means I have been kidnapped from my boat in the flotilla by the Israeli occupying forces, and I'm now being held illegally in an Israeli prison.
"I am so proud to be taking part in this flotilla - it is the largest to date."
In an interview with TG4, President Connolly said that the incident happened in international waters.
"It's quite upsetting, and I'm very worried about her, and I'm also very concerned about her colleagues on board," she said.
President Connolly added that due to her "very busy" schedule on the day of her official visit to England, she "hadn't really had a chance to get details" in relation to her sister or others on the flotilla.
Cllr Bree said he has had "no communication whatsoever" with his wife since her detention.
While he does not know where his wife has been taken, Mr Bree said they "assume now she is on the prison ship, that is what they did on the last occasion when they abducted flotilla volunteers off the coast of Crete".
He said "no state has the right to police or occupy international waters", but this is what Israel is doing.
Mr Bree spoke with his wife at 9am this morning when she told him that Israeli military ships were approaching boats, including hers.
He said a short time later, communication broke down, and he assumed she and others threw their phones overboard to prevent the Israeli authorities from gaining access to them.
"The boat in which my wife was sailing was unarmed, its mission was peaceful and yet we know it was boarded with force by the Israeli military, its crew of international volunteers abducted, and its cargo was confiscated," said Mr Bree.
Watch: Moment Israeli forces boarded one of aid flotilla vessels
Social Democrats Foreign Affairs spokesperson Patricia Stephenson has criticised the Government's response to the detention of Irish citizens on the Gaza aid flotilla.
Ms Stephenson said Government condemnation of what has happened represent "empty words".
She said the Coalition must instead demand that Israel releases the Irish citizens who are detained.
"These are our citizens being picked off against international law. If this was any other country bar Israel acting with such aggression, would we be reacting with such inaction?
"As a country, we should renew our call for the suspension of the EU-Israel Association Agreement.
"We should proceed with the Occupied Territories Bill. These are real actions that could prevent this from happening and have a real impact on the Israeli state," Ms Stephenson said.
Israel's foreign ministry earlier said on social media that it will not allow any breach of its naval blockade on Gaza.
As Israeli military vessels intercepted the flotilla fleet, the Global Sumud Flotilla demanded safe passage for its "legal, non-violent humanitarian mission".
It called on international governments to stop what it called illegal acts of piracy on the part of Israel.
Karen Moynihan, head of the Irish delegation for the flotilla, who is not on board, said 15 Irish citizens were sailing as part of it.
The flotilla organisers could not confirm how many, if any, of the Irish sailors have had their ships intercepted.
The flotilla organisers also posted a video showing boats approaching their flotilla. One video featured Irish citizen Mikey Cullen.
Ships from the Global Sumud Flotilla had set sail for a third time on Thursday from southern Turkey, after earlier attempts to deliver aid to Gaza were intercepted by Israel in international waters.
In its statement, Israel also called on "all participants in this provocation to change course and turn back immediately".
The previous flotilla departed from Spain on 12 April. But Israeli forces intercepted vessels in that group, taking more than 100 pro-Palestinian activists to Crete and detaining two others in Israel.
We need your consent to load this rte-player contentWe use rte-player to manage extra content that can set cookies on your device and collect data about your activity. Please review their details and accept them to load the content.Manage Preferences
Additional reporting Conor Hunt, PA and Reuters
