Brexit has undermined one of the greatest achievements in modern Irish history, Fianna Fáil leader Micheál Martin has said.
This, he said in Belfast on Wednesday, was the development of close and constructive relations between unionism in Northern Ireland and nationalism in the Republic.
Stopping this damage from getting worse, and prioritising a rebuilding of North-South working relations must become a real focus once the final form of Brexit becomes clear, Mr Martin told a meeting of the Irish Association in Queen's University, Belfast.
"No-one can doubt that relations between the Government in Dublin and political unionism are at their lowest point in at least 20 years," said Mr Martin.
"There has been a return to rhetorical sniping which has had much more impact than various welcome but largely marginal cultural gestures," he added at the conference on "healing the wounds of Brexit".
The Fianna Fáil leader referred to how for the first time in over a quarter of a century, the largest party of unionism, the DUP, "actually refused to attend an all-party negotiation if the Taoiseach was present".
He said: "This step backwards into a defensive posture of 'this is none of your business' is a radical departure from the approach which achieved so much in the past.
"Could you imagine just how little could have been achieved if David Trimble or Ian Paisley had refused to talk to Bertie Ahern – or indeed if Bertie Ahern had said, as the Taoiseach has, 'it's not my job to deliver the Unionists'?" he asked.
"Let's put this very simply: Without a return to an atmosphere of trust, understanding and permanent engagement nothing can improve," said Mr Martin.
He added that while Brexit has been "terrible for relations we will only begin to form an agenda for rebuilding them if we start off by understanding that Brexit has been the final part of a steadily deteriorating relationship in recent years.
"Brexit is causing so much damage specifically because the foundations were already weakened by years of disengagement and the failure to fulfil the enormous potential of the Good Friday agreement," he said.
Mr Martin said that whatever the outcome of negotiations in Brussels, there must be a determined effort to address the political and economic realities of a fragile relationship.
"We must remember that the hurdles we have overcome in the past are much bigger than any we face today, but we have to fully acknowledge that they exist and that we have to act if we are to return to building close and constructive relations on our island," he added.
Mr Martin described the "complacency" of the British and Irish governments as one of the chief reasons for the deterioration in relations.
"Where the Major, Blair and Brown governments had shown a deep commitment to Northern Ireland's long-term progress, there was an apparent impatience in the Cameron government that they still had to spend time on the issue," he said.
"The change of government in Dublin also marked a new approach. Set-piece meetings continued but there was a clear disengagement and an expectation that it was time to just let the DUP and Sinn Féin get on with business," he added.
Mr Martin said an "absolute first step in healing the wounds of this fractious Brexit debate is to return to the spirit of deep, ongoing and substantive engagement between our leaders at all levels.
"Relations between the unionist community and Dublin have suffered badly and it is largely irrelevant who is to blame. What matters is that we return to the spirit which allowed immense progress in the past in overcoming historic misunderstanding, suspicion and division," he added.
Mr Martin said Northern Ireland "has never had the post-conflict development plan it so badly needs, and developing one should be a shared objective — prosperity for all should be a shared and non-sectarian objective".