Participation by the Ulster Unionist party in the North-South Ministerial Council has put an end to cross-Border co-operation "behind the backs" of unionists, Mr David Trimble said after he and three party colleagues sat across the table from their Southern counterparts.
The North's First Minister and leader of the UUP played down the importance of Monday's events and insisted the week's "enormously significant event" would take place on Friday when the British-Irish Council convened in London.
He said unionists would always be present at meetings of the North-South council, which will meet twice a year. "Where there is agreement, it will be with our agreement. On that basis we have something that marks significant progress," he added.
Mr Trimble said the British-Irish Council meeting would be revolutionary as it signalled an end to the "cold war" by bringing together all administrations of the British Isles.
The North's Minister of Enterprise, Trade and Investment, Sir Reg Empey, said that under the Anglo-Irish Agreement, unionists had effectively acted as spectators in their own country, with Irish and British ministers meeting "over our heads".
As unionist ministers, they would approach the council positively, he said. "We will work hard to make the institutions work. We seek genuine benefit from mutual co-operation." The four SDLP ministers spoke of their satisfaction that the "Irish dimension" had finally been introduced to the process. Mr Seamus Mallon, the North's Deputy First Minister, said the council and the implementation bodies had enormous potential. Ms Brid Rodgers, the North's Minister of Agriculture, paid tribute to the vision of the party leader, Mr John Hume, who had consistently advocated that both traditions and both parts of the island work their "common ground".
The two Sinn Fein representatives, Mr Martin McGuinness, the Minister of Education, and Ms Bairbre de Brun, the Minister of Health, stressed the historical importance of the day and the dawning of a new political reality on the island of Ireland.
Mr McGuinness said his party intended to bring a radical and republican perspective to the new structures. "The importance of thinking, planning and acting on an all-Ireland basis cannot be overestimated," he said.
"I believe that by working together our common interests will come to the fore and that we can embrace the new opportunities which face us. This can be and I hope is the beginning of a new era for all of Ireland."