Adrian O’Neill hit out at UK Brexit negotiators for failing to provide a “firm commitment” over the sensitive border issue.
Theresa May has insisted the UK wants to avoid the re-introduction of a hard border with Ireland after Britain leaves the EU.
But some Brussels officials fear a ‘no-deal’ Brexit could put the Irish peace process at risk.
Speaking at the Centre for European Reform, Mr O’Neill said Dublin needed more assurances from Mrs May’s government.
He said: “We want to ensure that the process of co-operation between the UK and Ireland can continue in a dynamic way after Brexit.
“Maintaining a land border that is open and invisible is absolutely essential, politically, economically, socially and symbolically.
“A no-deal or a cliff-edge Brexit is in nobody’s interest.
“Genuine progress has already been made on some fronts in the Brexit discussions.
“But in regard to the land border, we have yet to find the flexible and imaginative solutions which are needed.”
“We certainly welcome the British Government’s stated objective of avoiding physical border infrastructure.
“However, to ensure the outcome we all want to see, we still need more assurance from the UK Government.
“What is needed is a firm commitment from the UK that the final outcome will maintain the openness and invisibility that characterises the border today.”
The ambassador’s comments came hours after the Irish foreign minister accused London of lacking “credible answers” to the issue.
Simon Coveney said: “We are trying to protect a peace process that so many people from all backgrounds, unionist and nationalist, have worked so hard to create.
“We are asking the hard questions and unfortunately we are not getting credible answers, which is why I think some people seem to be uncomfortable.”
Mr Coveney also insisted the Irish government’s position was “credible” and “consistent”.