The carefully drafted treaty was to have saved key parts of the EU draft constitution that failed when voters in France and The Netherlands rejected it in 2005. Ireland is the only country of the 27 EU members that requires a referendum vote to approve treaties. Voter turn out was low.
"We take note of the democratic decision of the Irish citizens with all due respect, even though we regret it," said a joint statement by French President Nicolas Sarkozy and German Chancellor Angela Merkel. Noting that the Lisbon Treaty had been ratified by 18 of the EU's 27 member states, the leaders added: "We therefore hope that the other member states will continue the process of ratification."
Environmentalist Green party chief Reinhard Bütikofer said the vote was a new "set back for Europe's ability to act."
Conservative Christian Democratic member of the European Parliament Elmar Brok called for continued efforts for ratification, saying the treaty would benefit EU member states and citizens. "This is why the ratification must go forward with out delay," he said.
The Left party leader Lothar Bisky called for a "pause for thought" in Europe so that leaders can consider how a social, civil and democratic Europe "can finally be legally founded."
Social Democratic MEP and head of the parliament's constitutional committee Jo Leinen said the with Ireland's rejection of the treaty, the EU has reached a "terrible crisis," that must be solved at the EU summit in the coming weeks.