Mary Robinson, who became the first county president when she was elected in 1990, said that if the customs posts were re-erected, it would be a "real risk" to the peace process.
He endorsed Taoiseach Leo Varadker's warning of a return to the period of violence that shook the region from the late 1960s to the 1990s, known as The Troubles.
This week, Mr. Varadkar was accused of alarmism after he said at an EU summit that violence could break out at border crossings if the UK fails to reach an agreement on Brexit.
Speaking to the Press Association at the One Young World summit in The Hague, Ms. Robinson said: "I think there is a real risk, if we had a hard border in Ireland, that there are people, as we would say, in the grass. long, on both sides waiting to make mischief and, unfortunately, could lead to that.
"We have had a peace process and a result of peace that the world has admired, it is really important that, on the island of Ireland, we live in peace, friendship and harmony, and that is what we have been doing."
"And I think it's really important that we do not have a border that provides a visible opportunity for those who could not only be harmful but also return to violence."
He added: "Most of the people I have spoken with in Northern Ireland believe this."
Spokesman for the Brexit of the Democratic Trade Union Party, Sammy Wilson, approached Mr. Varadker for sharing a story about an IRA attack with his fellow EU leaders in Brussels on Wednesday, calling it "vile."
He accused Taoiseach of using the stories of victims of terrorism to scare terrorists.
The bomb attack that occurred in a customs office in Newry, Co Down, in 1972, killed four customs officials, two truck drivers and members of the IRA.
The city was a bastion of the terrorist group and the attack occurred during one of the bloodiest years of the Problems.
The comments of Mr. Varadkar were defended by Tanaiste Simon Coveney, who said that people were really concerned about the destabilizing effect that a Brexit without treatment could have on the "very precious peace of the island".