Stormont’s deputy First Minister has accused the UK Government of treating the devolved institutions with “contempt”.
Sinn Fein’s vice-president claimed proposals for a bridge between Scotland and Northern Ireland were about diverting attention from the UK’s “reckless” approach to Brexit.
She said they would be better off spending any money earmarked for an Irish Sea span on boosting healthcare staff pay.
Ms O’Neill said: “The bridge is a pipe dream and it is more of an (act) to create a smokescreen in terms of the British Government’s reckless and partisan approach to Brexit that is all about deflection.”
The UK Government’s Ministry of Housing, Communities and Local Government (MHCLG) has said it will shortly recruit a small number of civil servants to be based in Northern Ireland.
This would help implement new UK-wide investment programmes.
A fixed link between Northern Ireland and Scotland could cost up to £20 billion to build.
A major transport connectivity review is assessing the feasibility of a bridge or tunnel.
The research is being carried out as part of Prime Minister Boris Johnson’s bid to improve UK transport links.
Ms O’Neill said: “The Tories have no mandate here. The Executive is elected to take decisions.
“This demonstrates contempt for powersharing and trying to interfere with what is a locally elected Assembly.”
First Minister Arlene Foster said the UK government should look across the country to see what is needed in terms of infrastructure.
She looked forward to engaging further with Sir Peter Hendy, who is carrying out a review of union connectivity, and his team that are now looking at the feasibility study.
Mrs Foster added: “It’s not just about the Boris Burrow or the tunnel, it’s much wider than that and I hope people will look at that because if the UK national government are looking at infrastructure across the UK that is a good thing.”
Stormont Infrastructure Minister Nichola Mallon has criticised the UK government for “concentrating power” in the review on the feasibility of the bridge.
Mrs Foster said: “I am somewhat bemused by the fact that our Infrastructure Minister doesn’t think it’s a good thing.
“You would have thought that the Infrastructure Minister would be welcoming the fact that the national government is focusing on infrastructure right across the UK.”
Sir Peter has said further work is required on the possibility of a “fixed link” across the Irish Sea.
Challenges include a post-war munitions dump off the coast of Scotland.
The most direct route for a span would involve crossing Beaufort’s Dyke, a deep submarine trench where an estimated one million tonnes of weapons have been jettisoned.
The Scottish Government has also dismissed the bridge as a vanity project.