The Dubliner backed about 150 Irish creative and performing artists who have already signed a pledge to boycott Israel until it ends its blockade of Gaza.
Dempsey said he believed the recent commando attack on the aid flotilla brought home the lawlessness of Israeli forces.
“What’s going on in Palestine is very wrong,” he said.
“We have a long history of oppression so I think anybody else who’s being oppressed, we should have a duty. And as artists we yield a bit more power than ordinary people. We have a duty to speak out and do something about it.
“I just want to make a stand for the Palestinians and show them some solidarity and show them they’re not alone.”
The pledge – organised by the Ireland Palestine Solidarity Campaign (IPSC) – states artists will refuse to perform or exhibit in Israel or accept funding from any institution linked to its government until it complies with international law.
Christy Moore, Donal Lunny, Robert Ballagh, Eoin Dillon and Sinead Cusack are among the novelists, playwrights, poets, actors, composers, singers, dancers, painters, sculptors and filmmakers involved.
However Dr Raymond Deane, IPSC cultural boycott officer, revealed it was the more unknown artists who were taking the biggest stand.
“The bravest of all are the ones you’ve never heard of because they are the ones who really want to make it and if they want to make it, you want to make it in the United States and they’re taking a risk. I think that’s very courageous,” he added.
Uillean piper Dillon – whose brother Shane was seized by Israeli authorities during the attack on the aid flotilla – performed a melody called Rachel Corrie at the launch in Temple Bar’s Meeting House Square.
He composed the simple tune in honour of the American college student who was crushed to death by a bulldozer in 2003 while protesting against Israeli house demolitions in Gaza.
He said he was shocked and disgusted at the conditions the Palestinians live in, with children denied basic health, education, food and shelter.
“It’s just intolerable, it shouldn’t be allowed,” he added.
“The governments of the world are not doing anything concrete. We’ve seen with the flotilla before that civil action can actually lead to something. It can get attention, it can lead to concrete results.
“It’s like South Africa, maybe the more people that will become aware of it, something will change.”