DMcG,
It was a Whitehall proposal and reviewed in many papers.

No. It was supposedly in a document leaked to the FT, a notoriously pro-Remain paper. The other papers only quoted the FT.
It was just a scare story. NI is not dependent on the Republic for its electricity.

The "barges in the Irish Sea" bit should have rung alarm bells. NI has nothing but space.
Why would they do that rather than put them somewhere more convenient to link to the grid?
It was all bollocks. Just more project fear.