"What emerged during the rancorous three-way negotiations among the U.K., Ireland, and the EU was the baffling 'backstop,' a contrivance to keep the Irish border open. During the transition period scheduled to begin March 29, 2019, it would be Britain’s responsibility to solve the border problem [otherwise] the backstop would come into effect.
Ireland Has Remarkable Power Over Brexit
https://www.theatlantic.com/ideas/archive/2019/03/ireland-has-remarkable-power-over-brexit/584434/