In 1916 the General Post Office in O'Connell Street, Dublin, served as the headquarters of the bloody Easter Rising against British rule. Its neo-classical columns are still pockmarked, allegedly with bullet holes.

In recent days scores of activists have been gathering outside that fine old building, now a symbol of Irish nationalism, to wage what they regard as a battle to stop the Irish Republic surrendering its independence to Brussels. They have been handing out leaflets, broadcasting messages and accosting passers-by, urging them to vote “no” in a knife-edge referendum on the European Union's Lisbon “reform” treaty.

They have not lacked encouragement. Ireland, bound by its constitution, is the only one of the 27 member states to hold a referendum on the treaty — the rest have preferred the safer route of parliamentary ratification — and citizens from other countries have rushed in to lend support.

Lony Ackermann, 68, a retired fire protection consultant from Berlin, has spent three days marching up and down the pavement with a hand-held megaphone exhorting Irishmen to vote “nein”. Thomas Rupp, 45, from Frankfurt, is part of a group that has erected cardboard cutouts of Angela Merkel and President Sarkozy with their hands over their ears and a large banner declaring, “Congratulations Ireland: You are having the referendum 486 million Europeans have been denied”. He was here, he said, “to make sure the Irish people understand they don't have to vote ‘yes'. It's not Europe's citizens who want them to vote ‘yes'. Only the elite want them to vote ‘yes'.”

Tourists from elsewhere in the EU have also expressed appreciation. Alami Farid, 44, from Paris, said: “I feel cheated. Sarkozy decided by himself to say ‘yes' to Lisbon. He didn't ask the people. He knew he would lose. I want the Irish to vote ‘no' and save us.”

Ian Norris, a hospice fund-raiser from Bury St Edmunds, also hoped the Irish would vote “no”. “Everyone in Europe should have been given the chance to vote, but the politicians didn't want to do that in case we gave them the wrong answer,” he said.

Even more encouraging for the activists outside the GPO, however, were the last two opinion polls of the campaign. One showed the “yes” camp's once-sizeable lead had been whittled down to three points, while the other, a real shocker in The Irish Times last Friday, gave the “no vote” a five-point lead.

The “yes” camp has fought back hard this week, but that Ireland should again be flirting with rejection of a key EU treaty - it rejected the Nice treaty in 2001 - is remarkable.

This is a country that, thanks to 35 years of EU largesse, has been transformed from an agrarian backwater into one of Europe's richest states; a country whose membership of the EU has enabled it to escape finally from Britain's shadow; a country where the entire political, media, business and trade union Establishment has backed the treaty against a bizarre ragbag opposition of maverick businessmen, right wing Roman Catholics, socialists, communists, pacifists and anarchists.

A “no” today would scupper a treaty that requires ratification by all 27 members and that has been renegotiated once already after the French and Dutch rejected an earlier version in 2005. It would trigger a fresh crisis of confidence and legitimacy in Brussels, and spark a new bout of institutional navel-gazing as the EU faces much more urgent challenges. It would wreck President Sarkozy's grand plans to appoint the EU's first president and foreign minister when his country holds the rotating presidency later this year. It would be deeply embarrassing for Gordon Brown, who refused to let Britons vote on the treaty. It would also be a severe blow to Brian Cowen's credibility a month into his tenure as Irish Prime Minister. He would be dubbed Gordon Cowen, one Irish journalist quipped.

Preoccupied with corruption allegations against the former Taoiseach Bertie Ahern, the Irish Government's lacklustre campaign has been outshone, outpaced and outspent by a “no” campaign lavishly funded by the mysterious businessman Declan Ganley. Irish Europhilia has waned as EU funds have dried up — “Eaten bread is soon forgotten,” said Michael Marsh, a political science professor at Trinity College Dublin.

But the Government's fundamental problem is that the treaty is 346 pages of dense legalese - the exact opposite of what the EU's heads of government called for in their Laeken declaration of 2001: an easy-to-read document that would simplify the EU's workings, agree a division of powers between Brussels and member states, and render the EU project intelligible to its citizens. The Times found not one voter on the streets of Dublin who had read the treaty, and even Mr Cowen admits he has not read it all.

This treaty contains no big idea like enlargement or monetary union, just the amorphous concept of making an enlarged EU work better. “What on earth is it about?” asked Dr Marsh.

Unable to campaign on the treaty's merits, the “yes” camp has resorted to dire warnings. The message has “not been about how good the treaty will be for you, but how bad it will be for you if you vote ‘no',” Dr Marsh said.

Ireland would be isolated in Europe and marginalised in Brussels, its leaders contend. Foreign investors would, they say, be deterred at a time of rapid economic contraction. “Future generations will not thank us if we are the ones who bring to a halt a union which has been the greatest force for peace and prosperity in our history,” Mr Cowen said.

The opposition, by contrast, has cherry-picked “timebombs”, real or imaginary, buried within the treaty. They assert, rightly, that Ireland would lose voting weight and have a commissioner — an important protection for small countries — only ten in every fifteen years. They argue, less credibly, that the treaty would threaten Ireland's military neutrality and the 12.5 per cent corporate tax rate with which it has attracted so much foreign investment. They claim, rather wildly, that Ireland would be forced to offer abortion — even euthanasia — on demand. The “yes” camp accuses its opponents of lies and distortion.

Nearly a third of respondents told The Irish Times that they did not know what they were voting for, and the suspicion has grown that the electorate is being hoodwinked by a devious political establishment. “There's a sense we're being railroaded. They're holding a big stick over our heads and saying: ‘You be good boys and get into line'. But the more you're threatened the more you're inclined to vote ‘no',” said Michael King, 54, a taxi driver. James Brennan, 40, a businessman, agreed. “They think people are idiots, that we have to do it because they say so.”

Even so, bookmakers were still predicting a narrow “yes” vote, and seasoned observers say the Government has been shaken from its torpor by The Irish Times poll and pulled out every stop. For example, Mr Cowen has bought off the farming lobby by promising to veto any decision from the World Trade talks that could damage Irish agriculture.

Whatever the result, three million Irish citizens will be voting today on behalf of nearly half a billion Europeans. Martin Reznicek, a Czech television reporter filming outside the GPO this week, summed it up: “It's up to the Irish to decide which way Europe is going to go.”

The road from Lisbon

—The Lisbon treaty was drawn up to replace the draft European constitution, which was rejected by voters in France and the Netherlands in 2005

—The treaty was signed by leaders of the EU's 27 member states in Lisbon last December

— Governments have been asked to ratify the treaty this year

— It is intended to come into force in January 2009

— Hungary was the first of 18 countries to ratify the treaty

—The British Government has resisted pressure to join the Irish Republic in holding a referendum

—The treaty cannot come into force if any state fails to ratify it

Source: Times archives

Times Recommends

• No apology for Germany’s own Nazi victims
• Intricate web of fear and hope tying down deal
• Victims remembered a year after Mumbai attacks