Ireland faces financial penalties from Europe if it systematically fails to enforce customs controls with the UK should the latter leave the Customs Union, a former European Commission official has warned.
Customs expert Michael Lux said Ireland would be obliged to have border controls in place - even if the UK chooses not to.
Mr Lux told a Westminster committee that member states which failed to enforce customs rules on EU borders faced having to cough up any lost duties.
"If the member state systematically disregards the rules, as a consequence duties will be evaded, the European Commission will calculate the duties, and make the member state pay any evaded duties.
"This is very serious," Mr Lux told the Northern Ireland Affairs Committee.
However, Taoiseach Enda Kenny last night said the Government "will not tolerate" any checkpoints or return to old-style borders between the Republic and Northern Ireland.
Mr Kenny made the remarks at a meeting of the Fine Gael parliamentary party where he updated TDs and senators on the Irish response to Brexit.
Mr Kenny also said there is no question of a Brexit Minister being appointed, arguing that "every minister and minister of state" is working on it.
But Mr Lux, a customs and international trade lawyer, said that if goods were brought from Ireland to the North after the UK leaves the Customs Union, an export declaration would have to be lodged on the Irish side.
Mr Lux was interrupted by Sylvia Hermon, a Northern Ireland MP, who pointed out there were currently no officials at the Border, and that Prime Minister Theresa May had stressed there would be no return to the "borders of the past".
"Yes, but Ireland has to do so," said Mr Lux. "What you do on your side, that's up to you. If you feel that you don't need to control the goods that are entering Northern Ireland… you're of course free not to do that. But Ireland is obliged to do this."
Asked if a "seamless, frictionless" Border was achievable, Mr Lux said it all depended on how "seamless" was defined.
"If you define the term 'seamless that there are no border controls, then the answer is no, at least for the side of Ireland, because Ireland is obliged to apply the union law."
He said there would have to be customs officials on the side of the Border in the Republic.
"Even if they don't like it, they will have to do it," he said.
He suggested achieving a special arrangement between the UK and Ireland on customs would be "problematic".