Building a bridge or tunnel between Scotland and Ireland may seem far-fetched to anyone who has made the ferry crossing in stormy weather.

But such proposals were first mooted more than a century ago and continue to attract debate.

East Antrim MP and leading DUP member Sammy Wilson last year compared the importance of building a fixed crossing over the North Channel of the Irish Sea to the high-speed rail project underway in the south of England.

He called for a feasibility study on the subject, and added the idea of building the Channel Tunnel to France was once widely derided.

A 2007 report by the Centre for Cross Border Studies estimated building a bridge from Galloway to Ulster would cost £3.5 billion.

Group director Andy Pollak envisaged a rail link allowing passengers to board trains in Glasgow and alight in Belfast or Dublin.

“It struck me that the gains to be obtained from building a bridge which might cost a couple of billion across the North Channel between Scotland and Ireland would outweigh the cost,” said Mr Pollak at the time.

There are two potential routes regularly suggested for a crossing. A Portpatrick to Belfast Lough link would be around 21 miles in length, while Antrim to Mull of Kintyre just 12 miles.

The latter option is routinely discounted however, as the road network from Campbeltown on the Scottish side would require significant upgrading through mountainous terrain, and lacks a direct rail service.

But any bridge or tunnel between the two countries would face other logistical challenges.

One of the biggest obstacles is Beaufort’s Dyke, a 31-mile long sea trench more than 200m deep.

It lies six miles from the Scottish coast and was used as a dumping ground for conventional and chemical munitions in the aftermath of the Second World War.

Bridges covering longer distances have been built elsewhere, but engineering experts are sceptical that such a crossing could be built over the North Channel.

Ronnie Hunter of the Institution Of Civil Engineers said a rail tunnel would be a more likely option.

“The North Channel is something like 32,000 metres coast-to-coast, where as the Forth Road Bridge is around 2,500 metres in comparison,” he said.

“If you’re talking about building a bridge, it would be multi-span and require dozens of piers across the channel.

“There are numerous bridges in North America built across relatively shallow water which go on as causeways for mile after mile. But we’re not talking about shallow water here - this is essentially next to the Atlantic Ocean, in very deep water.”

Hunter, who has more than 40 years’ experience in civil engineering, added the financial cost of building a tunnel would be prohibitive.

“The length suits a tunnel,” he said. “It would likely have to be a rail tunnel, rather than a road tunnel, as it is hard to get the ventilation right.

“One slight problem, which could easily be overcome, is that track gauge is different in Ireland to the UK.

“But what you have to ask yourself is, is there a benefit in building such a link? Someone is going to have to face a substantial cost, and it’s not clear to me you could make that cost-benefit calculation work.”

Another obstacle to building an Ireland-Scotland link is a lack of official support at government level.

John Wilson, then Irish minister for tourism and transport, estimated in 1987 that an Irish Sea tunnel would cost twice as much as the Channel Tunnel but generate only one fifth of the revenue, rendering it economically unviable.

A spokesman for the Scottish Government said it recognised the importance of strong transport links between Scotland and Northern Ireland, but stressed these were limited to flights and existing ferry routes.

“We very much welcome the investment that both P&O and Stena have made improving the ports and vessels on the ferry routes from Loch Ryan to Larne and Belfast in recent years,” the spokesman said.

“We are supporting this through the continuing maintenance of the trunk road network to allow people to reach the port safely and efficiently with £58 million invested in the M77 and A77 since 2007.”

This article first appeared in The Scotsman.