A withdrawal treaty including any Irish border deal will have to be agreed by October, Taoiseach Leo Varadkar warned.
The deadline was necessary to allow time for UK and European parliaments to ratify arrangements before Brexit next year, he said as he arrived at a European Council meeting of state leaders in Brussels.
Mr Varadkar reiterated his firm stance on the importance of the backstop arrangement in which Northern Ireland would continue to follow EU regulations which affect the island if no other solution is found.
But he said that was not his preferred option.
"So the way it can be changed is for us to develop option A, which is a really close trading relationship between the UK and the EU, so close that many of the things that are in the backstop may become unnecessary, if that makes sense," he said.