The boat had set off from Cyprus on Monday, carrying humanitarian supplies for Gaza, which remains under an Israeli blockade.
Speaking to Al Jazeera, Caoimhe Butterly, a spokesperson for the Free Gaza movement, said: "This is the third humanitarian relief boat and solidarity mission that has been aggressively stopped by the Israeli army.
"The humanitarian situation is at crisis point. There has been a systematic and deliberate destruction of the humanitarian infrastructure of Gaza.
"We were taking medical supplies and medical equipment and limited amounts of building material ... although $4 bn has been pledged to the Gaza Strip for reconstruction ... no cement or building supplies are allowed in as a result of the siege.
"We need to up the ante and to mount more actions and convoys of this nature," she said.