More than 50 boats sailed from Marmaris, Turkey, last week with the goal of breaking Israel's blockade of the Gaza Strip.
The Global Sumud Flotilla's live broadcast showed activists raising their hands in the air as Israeli soldiers were about to board the flotilla. Before the intervention, Israel had urged the activists to "change course and turn back."
The president's sister and doctor, Margaret Connolly, is one of eight Irish people detained by Israel, according to a statement from the activists.
Catherine Connolly, speaking during a visit to London, says she is "very worried" for her sister and the other activists.
