LONDON — A day after Prime Minister Boris Johnson presented his plan to lead Britain out of the European Union — a dense farrago of details about customs unions, regulatory zones and borders running in two directions — Ireland’s prime minister, Leo Varadkar, offered a simple response: Why not just vote again?

“All the polls since Johnson became prime minister suggest that’s what the British people actually want, but their political system isn’t able to give them that choice,” Mr. Varadkar said on Thursday during a visit to Sweden.

The spectacle of an Irish leader opining on the dysfunction of British politics — not to mention brushing aside Mr. Johnson’s oft-repeated vow to carry out the will of the people by delivering Brexit — does not sit well with many in Britain. But there is little they can do about it.

Mr. Varadkar, a 40-year-old physician-turned-politician, holds the crucial vote on whether Mr. Johnson’s plan will pass muster with Brussels. Unless Mr. Johnson can satisfy him that it will not disrupt the fragile peace in Northern Ireland, it is highly unlikely that he will be able to persuade the other 26 members of the European Union to accept it.