A withdrawal treaty including any Irish border deal will have to be agreed by October, the Taoiseach Leo Varadkar warned.
The deadline was necessary to allow time for UK and European parliaments to ratify arrangements before Brexit a year from now, he said as he arrived at a European Council meeting of state leaders in Brussels.
Much of the remaining work remains on the vexed issue of the Northern Ireland border with the Republic of Ireland where there is relatively little detailed operational agreement.
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 envisaged a deep free trade agreement between the UK and EU.
“Or for example, what has been talked about before, a customs union partnership between the UK and EU that would be so close to the customs union that it would not necessitate some of the elements that are in the backstop.”