Ireland was "seeking legally binding instruments to address the concerns of the Irish people", a government spokesman told the BBC.
Once it got those assurances, it would present "a roadmap for ratification", that would include another referendum, the spokesman added.
Hans-Gert Poettering, president of the European Parliament, said a wide-ranging consultation with the Irish people was needed. He said dialogue with Irish citizens before the first vote in June "was not serious enough".
"We hope for a solution by the end of next year," he said. "Realistically it won't be the case before the European elections in June."
The EU is set to offer guarantees that the treaty will not affect three main areas of concern to Irish "No" voters - abortion, Irish neutrality and taxation, says the BBC's Europe editor Mark Mardell.
Ireland is also likely to be able to keep its EU commissioner.
Barroso on Ireland's EU vote
But Declan Ganley, the chairman of the Libertas group that led the No campaign in the first Irish referendum, said this was an example of the Irish being dictated to.
"Do we think that democracy is important in Europe or do we want to exist in some post-democratic environment where European affairs are concerned?" he said.
He also announced that his Libertas group would be standing on an anti-treaty platform across the EU during next year's parliamentary elections.
Credibility at stake
EU leaders will pore over and work on the summit conclusions before they are published on Friday.
They also face a major test of their willingness to tackle climate change, with a key agreement on cutting the EU's carbon pollution at stake.
Mr Poettering said the parliament needed to find a compromise.
"It wouldn't help anyone if industry simply relocates to China or somewhere," he said. "The important thing is that Europe remains in front on this."
Amid the economic downturn, Germany, Italy and Poland, among others, are fighting any deal that could cost jobs.
Poland's Europe Minister Mikolaj Dowgielewicz said there would be long negotiations over the package, including Poland's proposal for a central "solidarity" fund to help poorer nations cope with CO2 measures.
"It's going to be a three or four-shirt summit. We've booked tickets for Sunday morning..." he told the BBC.
"There's a number of issues which are still open, everything from the financing of CCS [carbon storage] to the solidarity fund - this is still on the table... we don't really know what the compromise will be."
EU 20-20-20 TARGETS
20% cut in greenhouse gas emissions by 2020
20% increase in use of renewable energy by 2020
20% cut in energy consumption through improved energy efficiency by 2020
The "20-20-20" package, which also requires approval by the European Parliament to become law, commits the EU to cutting carbon dioxide (CO2) emissions by 20% by 2020, compared to 1990 levels, and to raising renewable sources to 20% of total energy use.
President Sarkozy is pushing hard to clinch a deal before he hands over the rotating presidency of the EU to the Czech Republic at the end of the year.
European Commission President Jose Manuel Barroso said: "It would be a real mistake for Europe to give the signal that we are watering down our position, after all these years leading the efforts for a global solution."
Also up for discussion is the EU's 200bn-euro (£175bn) economic stimulus plan.
With recession looming, there will be broad agreement on the EU-wide package to boost the economy, although Germany opposes calls from Britain and France to cut taxes, says the BBC's Oana Lungescu in Brussels.
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