Irish voters rejected the reform treaty last week, raising questions about the EU's future and casting a pall over a two-day summit of the bloc's leaders that begins later on Thursday in Brussels.
Speaking in the Bundestag lower house of parliament before heading to the summit, Merkel said Europe needed the Lisbon treaty but rejected the notion that Ireland could be excluded following the "no" vote.
"We must ensure that treaties in the European Union are brought forward unanimously. There's no other way," she said.
"Therefore discussions about a two-speed Europe, a core Europe, do not help us at all. I think this discussion does not help us achieve our goals and is even negligent. One cannot have an enlarged European Union and then whenever a problem pops up, say we should have a core Europe," she added.
Merkel expressed confidence the 27-nation EU would find a solution to the impasse.
"I am deeply convinced and resolved that we must find a solution to this situation together with the Irish. And we will use all our power to push for a common solution," she said. "I am also convinced that we can and will find a solution."