STRETCHING for 21 miles, it would span one of the last major expanses of water separating European states and allow rail passengers to travel from Glasgow to Dublin in about 90 minutes.
An Irish think tank has proposed that Ireland should join the European high-speed rail network, via a new bridge or tunnel to Scotland.
The Centre for Cross Border Studies admitted the link would be a "hugely ambitious" project, but predicted it would provide major economic benefit to both countries.
Andy Pollak, its director, said a bridge was more likely than a tunnel across the North Channel between County Down or County Antrim and the Mull of Galloway, near Stranraer.
However, he was unable to say how much it might cost. A new one-and-a-half mile bridge across the Forth is estimated to cost £1.5 billion, while a four-mile tunnel - the shortest option - would cost £2.1 billion.
Mr Pollak said the link could be built by 2030, when trains travelling at 187mph could reach Paris from Dublin in seven-and-a-half hours.
Mr Pollak said: "A link would provide a massive boost to economic and social links between both parts of Ireland and Scotland, something a lot of people, including the Taoiseach and both the Northern Irish and Scottish First Ministers, view as an unadulterated good."
Alex Salmond, the First Minister, discussed improved transport links when he met Ian Paisley, his Northern Ireland opposite number, in June.
Mr Pollak said the project would also boost Ireland's trading opportunities with Europe and relieve pressure on Ireland's overloaded airports.
He admitted the plan would also require costly upgrading of the 100-mile Stranraer-Glasgow rail line, which is protected under European law because it is already classed as a trans-European route.
Mr Pollak said the original idea was that Ireland should be linked into the European high-speed rail network, which stretches only as far north-west as Glasgow
He said: "I put it forward because I think it could be one of the triggers to the next phase in Ireland's economic development. And who's not to say that it could not also be part of the next phase of Scotland's economic development?"
However, the scheme was met with a muted reaction in Scotland yesterday.
A spokesman for the Executive said: "While there are potential economic benefits to both Scotland and Ireland in building better transport links, we have no current plans for a bridge."
The North Channel Partnership, which comprises local authorities and business groups in Scotland and Northern Ireland, said the 1,000ft deep Beaufort's Dyke trench could pose problems. It also warned about the impact on the current £40 million redevelopment of the Cairnryan ferry terminal. The port will handle the lion's share of the two million annual Scotland-Northern Ireland ferry passengers when Stena Line moves there from Stranraer to join its rival,
P&O Ferries, next year.
Robert Higgins, the partnership's chairman, said of the link plan: "It's an interesting idea, but if anyone is doing some blue-sky thinking on this then the challenges have to be weighed up against the perceived benefits, not least the position and depth of Beaufort's Dyke.
"Also, the ferry companies are investing in the port currently so the implications there also need to be considered."
Mr Pollak's proposal is the latest in a series of such schemes that stretch back more than a century. Test borings for a tunnel under the North Channel were considered in 1895, and several politicians in the British and Irish parliaments have since called for a link. A tunnel under the Irish Sea to Wales has also been proposed.