SUDDENLY IT'S a contest, and a momentous one. A massive late swing toward a No vote in Ireland's inconvenient referendum on the EU treaty this Thursday has overturned an assumption that a Yes was in the bag.

The Irish are full of surprises. Dublin's disproportionate influence on the EU comes from top-quality schmoozing within the Brussels network, rather than a tendency to make principled protests outside of it.

Given that the Lisbon treaty requires complete unanimity among all 27 member states, this is extremely serious for every member state. Memories of 2001, when Ireland shocked the union by rejecting the Nice treaty, have flooded back. This time they can't just let voters have another shot at getting the right answer.

Gone is the patronising assumption that the Irish, while not necessarily subservient to Brussels, have always known on which side their bread was buttered. They didn't look likely to scupper seven years of painfully won progress towards "streamlining" the EU.

The only member country constitutionally required to hold referenda on major treaties, it was assumed that the Irish would ultimately do the right thing. Lisbon after all is already Plan B. There is no Plan C.

In the teutonically grim words of German think-tank Bertelsmann Stiftung a rejection would be "a catastrophe ... a serious setback for the European constitutional treaty and the future of the EU."

Where does this leave our fellow Celtic nation? Should this quirk of Irish democracy deprive the union of its chance finally to "tidy up" its executive processes (a sinister concept to many), it would condemn Europe to further years of tedious and expensive negotiations. Former Irish Taoiseach Bertie Ahern, before he left office under a cloud of his own, said a No vote would leave Ireland "whistling in the dark", though in truth few other Western European member states could secure a Yes vote if they dared to hold their own referenda.

This consensus was strengthened by the fact all of the Irish political parties, with the exception of the fringe-dwelling Sinn Féin, are campaigning strongly for a Yes vote. The same goes for Irish business, and representative groups such as IBEC, the Irish equivalent of the CBI. A Reuters poll of Dublin-based economists last week found that nine out of 10 of them were convinced the best way to help ensure Ireland's prosperity was to tick the Yes box.

So why isn't this a shoo-in?

Grassroots defiance of the elite consensus, as well as playing the familiar Eurosceptic themes, shows how little was learned by the Nice treaty rejection. Then, as now, the problem seemed to be the Yes camp's inability to explain to the electorate what exactly they were being asked to decide, and why it mattered. Now as then, the simple slogan, "If you don't know, vote no" is proving devilishly effective.

While there may be strong political reasons for Ireland to desist from throwing cold water on the reform treaty, the best of the economic arguments for a Yes vote is hardly a ringingly positive one, though it does speak to the vital importance of low tax in the modern Irish story.

As Rossa sic White, chief economist of Dublin stockbrokers Davy puts it: "Now more than ever we need to safeguard our veto over corporation tax changes. That veto remains enshrined in the Lisbon treaty, so it is important that we ratify it."

Translation: by voting to strengthen the ability of the big nations to impose their will on smaller ones (as the new not-a-constitution clearly does), the Irish will somehow be increasing their power to resist that will. It is a tortured argument to say the least.

In fact there is nothing even in the copious small print of the Lisbon treaty about direct taxation or corporation tax that suggests they will move out of the exclusive province of the member states. But that fact does not give much reassurance in the slippery world of the European Union.

The French, who begin their presidency of the EU next month, have been pretty explicit about their desire to push for the harmonisation of company tax, a useful piece of ammunition for the Irish No camp.

The French finance minister, Christine Lagarde, told a Brussels economic forum in April that the matter is "one of the most urgent" regarding European fiscal policy and is "certainly a question which we want to push,"

European Union taxation commissioner Laszlo Kovacs is expected to present a proposal for a common basis for company tax throughout the 27-nation EU - not to fix a standard rate but to harmonise the basis for setting rates - a subtle distinction that will be lost on many members. In order to increase competitiveness in the EU, Kovacs said: "There is a real need for the member states to act together in certain tax policy areas."

For those in Scotland who see the Irish example as a strong advertisement for the benefits of fiscal autonomy or more, the effect of an Irish No vote on the European project is especially interesting. No-one really knows what this effect will be, and no-one in authority in Brussels wants to be appearing to be making threats by discussing possible scenarios.

Scotland's potential status within the EU should be a vital aspect of the SNP's independence "conversation". A potentially serious future divergence of views between Dublin and Brussels on tax, is something Scottish nationalists should be raising.

And if France, Germany and others get their way and muscle through some degree of corporate tax harmonisation, where would that leave the newly independent micro-member Scotland? If the Irish vote No, resulting in a new era of fudge, then answers to such hypothetical questions, essential for an informed referendum on Scottish independence, will recede further towards the blurred horizon.

The SNP's most alluring promise to Scottish business - endorsed by objective observers like Mary Fulton of Deloitte or economists Ronald MacDonald and Paul Hallwood - is that fiscal autonomy would lower corporation tax and end psychologically debilitating and wasteful spending. It is an attractive and exciting prospect on many levels.

Or it would be, if the SNP government gave more indication that it knew how to lower public spending in alignment with the reduced tax revenues that would follow a corporation tax cut.

The lessons of Ireland are of limited use here, given that we have North Sea oil production (albeit steadily declining), but would lack the EU infrastructural pump priming that the Irish used so skilfully.

The real lesson of Ireland is that timing is everything. It is not yet clear that we have their proverbial luck.