Fianna Fail leader Micheal Martin has accused the Irish government of abandoning the peace process.
And Taoiseach Enda Kenny is risking consequences worse than already feared unless he makes more of an effort to resolve a brewing crisis, warned Mr Martin.
Speaking at his party's annual Wolfe Tone commemoration in Bodenstown, Co Kildare, Mr Martin said Dublin has failed to respond to a rise in sectarian rows, escalating tensions and a collapse of faith in institutions set up by the Good Friday Agreement.
"What has been missing is any serious engagement by either the Dublin or London government," he said.
"They have been complacent and disinterested.
"Particularly damaging is how the entire issue of development through North/South cooperation has been ignored."
Mr Martin criticised a recent development plan for Northern Ireland which "makes no mention of the Republic" and hit out at the Taoiseach for not attending a major investment conference in Belfast.
"Worst of all it is how vital North/South infrastructure projects such as the Narrow-Water Bridge are being abandoned because of a lack of commitment from the governments," he said.
"The economic and social development of this island, and of the border region in particular, demands strong North/South cooperation. "
The Fianna Fail leader said historic progress for peace and reconciliation on the island could not have been possible without the priority which different Irish leaders gave to it.
"Formal meetings and dinners aren't enough - you have to spend the time developing links and building trust," he said.
"The current Taoiseach needs to start giving it his attention or the consequences may be much greater than we already fear."
Really sorry 2 hear of the death of Albert Reynolds. Solidarity 2 Kathleen & all their family. Albert acted on North when it mattered. RIP.— Gerry Adams (@GerryAdamsSF) August 21, 2014
I liked Albert Reynolds. The Joint Downing Street Declaration was one of the defining moments in British/Irish history.— Simon Fraser (@TheSimonFraser) August 21, 2014