Any border infrastructure on the island of Ireland would be a "clear and present danger" to the Northern Ireland peace process, it has been warned.
Irish senator Neal Richmond said repeated warnings against any type of border between Northern Ireland and the Republic were not "alarmism".
The Fine Gael EU affairs spokesman warned: "Any border and customs infrastructure is a viable threat to the peace process which is only 20 years old on this island.
"The Irish Government is a co-guarantor of the Good Friday Agreement... as are the British government, so we do have a responsibility to make sure that case is known."
Speaking on ITV's Peston On Sunday he added: "It is not alarmism. We get tired of people saying we are fear-mongering. This is a clear and present danger to our peace process which needs to be tackled."
Last week the PSNI's Chief Constable George Hamilton warned that any infrastructure at the border would become a target for armed groups and pose a danger to his officers.
"The terrorists only have to be lucky once to get a result with catastrophic consequences," he said.
The EU's chief Brexit negotiator Michel Barnier said on Friday that a UK decision to leave the single market and customs union would make Irish border checks "unavoidable".
He made his comments after Downing Street said the UK would not be in a customs union after Brexit.