A lack of leadership is resulting in "drift" between people Northern Ireland and the Republic, according to Fianna Fail leader Micheal Martin.
Mr Martin said that separation between the two societies "is absolutely not inevitable and it can be reversed".
In a hard-hitting speech, he warned that the failure to take advantage of the opportunities from peace poses a long-term threat to what has been achieved.
He said: "We are failing to take advantage of the many and obvious opportunities which peace and a legitimate constitutional blueprint have brought.
"No one who knows anything about our history should think for a moment that there is nothing to be worried about."
In the opening address of the 2013 Brian Merriman Summer School in Lisdoonvarna, Mr Martin said: "There was nothing inevitable about the success of the peace process and there is nothing inevitable about its longer-term course."
He added: "Institutions are in place. Meetings happen on schedule. Speeches about how well everyone is getting on are delivered all the time.
"Yet there is absolutely no urgency or ambition."
Mr Martin blamed an absence of leadership for not improving relations, resulting in the two societies growing apart.
He added that the border region remains the most disadvantaged on this island.
He said: "The recent scrapping of North-South infrastructural projects is the worst thing that could be happening."
This was a reference to the Irish government's decision to withdraw £400m funding for the A5 dual-carriageway project. The road forms the main route from Dublin to Derry and Donegal.
This is not the first time Micheal Martin has intervened to criticise drift in the peace process. In May, he said Stormont's ruling political parties have created a dangerous vacuum by failing to fulfil the promise of peace and reconciliation. He also accused Sinn Fein and DUP of showing no interest in using their strength to bridge differences.