The Irish government's failure to support a former unionist leader contributed to the collapse of the 1974 Sunningdale Agreement, a former top-level diplomat in the Republic has said.

The short-lived power-sharing deal between the Ulster Unionists and the SDLP collapsed after just five months when an Ulster Workers' Council strike brought Stormont down.

A Council of Ireland proposed in the deal also never met but the biggest failure was not preventing further sectarian bloodshed in Northern Ireland.

Former SDLP deputy Seamus Mallon famously called the Good Friday Agreement of 1998 "Sunningdale for slow learners".

In a major new book on Anglo Irish relations by Noel Dorr, the former secretary general of the Republic's Department of Foreign Affairs, reveals that the Irish government at the time contributed to Sunningdale's downfall by not doing more to support the UUP leader Brian Faulkner.

Although laying most of the blame with the British Government and loyalists in Northern Ireland, Mr Dorr said the Irish government led by Taoiseach Liam Cosgrave and minister of foreign affairs Garret Fitzgerald had "more than a gleam in their eye" that the Council of Ireland would lead to Irish unity and did little to reassure unionist fears.

"Unionists still felt under siege and, understandably, many believed that the continuing violence showed they had nothing to gain and much to lose by the agreement," he said.

He also revealed the Department of the Public Service in Dublin had proposed assigning up to 20,000 civil servants to work on the Council of Ireland.

"If it had become public at the time it might well have sunk the concept of the council on the spot," he said.

Despite Sunningdale's failings, Mr Dorr said it succeeded in making the UK Government realise "the Irish question" had not been settled after partition in 1921.

He added: "If in future, there was to be a devolved government then representatives of the minority would share in exercising the devolved powers, and the aspiration of the minority to Irish unity achieved by consent would have to be recognised."

Sunningdale: The Search for Peace in Northern Ireland, is published by the Royal Irish Academy.

Belfast Telegraph