“The stars are aligning” for a bridge between Scotland and Northern Ireland, according to the principal advocate for a Celtic crossing, the leading architect Alan Dunlop.

Although engineering experts have dismissed the concept as “bonkers”, Dunlop has been pressing for serious discussion of Boris Johnson’s latest grand infrastructure scheme since he conducted a feasibility study into the proposal in 2018, when he first raised the prospect.

Basing his calculations on similar-sized structures, Dunlop estimates the project would cost between £15bn and £20bn.

“People don’t take it as seriously as it deserves because it is associated with a divisive politician,” Dunlop said. “You have to separate the architectural, economic and engineering merits from resentment towards Boris Johnson.”

But the day after the prime minister’s spokesman confirmed government officials were “actively looking into” the idea of a Scotland-Northern Ireland road bridge, and as Johnson announced to the Commons that the HS2 high-speed rail line would go ahead, Dunlop’s enthusiasm was tempered by pervasive local cynicism as well as frustration at the lack of investment in existing road and rail networks.

Two potential routes for such a bridge have been suggested– from the Mull of Kintyre to the Antrim coast and, now considered more likely, from the picturesque fishing village of Portpatrick to Larne.

Robert Erskine, a harbour master at Portpatrick, dismissed the scheme as “a nice pipe dream”.

Erskine, also vice-chair of the Portpatrick Community Benefit Society, which has worked to modernise the harbour since taking it into community ownership in 2015, said: “It sounds a wonderful thing but it’s no use unless you get the road infrastructure sorted out.”

A lifeboat coxswain at Portpatrick for nearly three decades, Erskine has an intimate knowledge of the often stormy channel between his village and Larne.

“It is a busy sea lane, deep water, heavy sea and then there’s the wind factor,” he said. “Over the past few nights, any bridge would have had to be closed. I can imagine a bridge being closed a lot, especially in wintertime.”

“A vanity project”, is how Catherine Branson of the A77 Action Group, dedicated to improving the main coast road to Portpatrick, described the bridge.

“We have no dual carriageway between [the current ferry terminal for Northern Ireland] Stranraer/Cairnryan and Ayr, which makes a joke of the futuristic bridge Mr Johnson proposes,” she said.

“There are poor rail links between Stranraer and the north, nonexistent rail between Stranraer and the south and no link between the ferries and the rest of the network. We don’t see the smokescreen of a bridge proposal taking any pressure off our determination to see real investment in the road infrastructure in the south-west of Scotland.”

A serious investigation will have to take into account Beaufort’s Dyke, a 300-metre-deep submarine trench where the Ministry of Defence dropped an estimated 1m tonnes of munitions after 1945, making it the largest known British military dump in existence. Locals describe old munitions, some containing phosphorus, being swept on to beaches as “a part of life”.

While Dunlop, a visiting professor at Liverpool University, acknowledged the depth and contents of Beaufort’s Dyke presented a “major challenge”.

He cited examples such as the Ǿresund Bridge between Copenhagen and Malmö, the world’s longest sea bridge connecting Hong Kong-Zhuhai-Macau, and the Norwegian coastal highway, which is 600 miles long and composed of tunnels as well as floating, suspension and cable-stayed bridges.

“No matter what the circumstances and geological challenges, there are solutions to overcome them,” Dunlop said.

The proposal, which appeared in the DUP’s 2015 manifesto, was dismissed on Monday as a “diversionary tactic” by the Scottish first minister, Nicola Sturgeon.

A local MSP, Joan McAlpine, said she would need to be convinced of the economic benefits for the region.

It risked becoming a crossing point for HGVs on their way to the central belt or the north of England without the local road infrastructure to support them, she said.

• This article was amended on 12 February 2020 because an earlier version referred to phosphorous, when the element phosphorus was meant. This has been corrected.