Joan Burton, who represents one of Dublin's constituencies for Ireland's third biggest political party, said: "It looks very much now like the 'No' vote are in the lead.
"I think that overall in Dublin most of the Northside working-class areas seem very much to be 'No' votes.
"There is quite a strong 'Yes' vote in some of the more middle-class areas but I honestly don't believe that it is strong enough to overcome the size of the 'No' vote."
If the treaty is not adopted, the 27-nation EU would face diplomatic turmoil since all EU nations must approve the negotiation pact for it to become law.
The complex document, a follow-up to the failed constitution that France and the Netherlands voted against in 2005, would reshape EU institutions and powers in line with the bloc's rapid expansion.
Ireland poses its greatest challenge, because all other EU members are requiring approval only through their national governments.
So far 18 EU members have ratified it, including the parliaments of Estonia, Finland and Greece.
The Irish government, major opposition parties and business leaders all campaigned for a "yes" vote during a month-long campaign that emphasised Ireland's strong benefits from 35 years of EU membership.
In a final appeal on Wednesday, Jose Manuel Barroso, the European commission president, said ratification by all states would allow the EU "to turn the institutional page and concentrate 100 per cent on delivering on the expectations of Europe's people."
Anti-establishment pressure groups from the far left and right mobilised opposition by claiming that the treaty's passage would result in Ireland losing control of everything from its business tax rates to its ban on abortion.
All the anti-treaty lobbyists argued that EU chiefs were trying to centralise power in Brussels by strengthening its senior positions and reducing the range of policy decisions that required unanimous support.
Many voters complained that the EU's near-doubling in size since 2004 had brought unwelcome change to Ireland, particularly the large influx of job-seekers from Poland and the Baltic states who fill in the majority of available jobs.