The undersea tunnel, which has been dubbed the ‘Boris Burrow’, would run from Portpatrick in Dumfries & Galloway to Larne in Northern Ireland
It would be modelled on the Channel tunnel, which is 31.5 miles long and runs underwater for 23.5 miles.
Sir Peter Hendy, the chairman of Network Rail, has been tasked with conducting feasibility studies into the proposal.
But Mr Cummings, in making a series of explosive claims in a tell-all interview with the BBC on Tuesday night, was particularly critical of the proposal.
The Prime Minister’s former aide said: “First of all he [Boris Johnson] was, he was fed up with the media portrayal of him being a kind of puppet for the Vote Leave team, it was driving him round the bend,” Mr Cummings said.
“I had a plan, I was trying to get things done. He didn’t have a plan … he didn’t have an agenda. You know, the Prime Minister’s only agenda is buy more trains, buy more buses, have more bikes and build the world’s most stupid tunnel to Ireland, that’s it.
“Also he knew that we basically disagreed about what was happening on Covid and he knew that I was blaming him for not having acted in September, which I was.”
Critics have described the proposal for the transport link as a “vanity project”, but Scottish Secretary Alister Jack has previously praised the the Union Tunnel idea as a “fantastic” idea.