Ireland feared a doomsday scenario could develop in British-ruled Northern Ireland in the 1970s if sectarian violence descended into all-out civil war, newly-released archives showed on Tuesday.

Dublin drew up a contingency plan amid concerns that over 250 000 Roman Catholics could be trapped and unable to escape, according to the previously secret files released by the national archives office.

There were also fears that if the British army withdrew, Dublin might be forced to deploy the Irish army, to prevent the Irish Republican Army (IRA) from taking up the role of "protectors" of the minority community.

Up to 200 000 Catholic refugees could flee south across the border into the Irish Republic, according to one file which suggested boosting Irish army and police numbers "without provoking an answering escalation" in Northern Ireland.

Northern Ireland, a province of the United Kingdom, was dogged by three decades of civil unrest known as the Troubles, which killed about 3000 people before Britain and Ireland brokered the 1998 Good Friday peace agreement.

In 1974, Dublin was worried the situation might deteriorate and Britain could re-assess its involvement in Northern Ireland after a five month old Catholic-Protestant power-sharing administration collapsed.

A foreign ministry study warned that, if a civil war broke out, some 262 097 Catholics were at risk in Belfast, north County Down and County Antrim where the Protestant population numbered 665 653.

"If a doomsday situation does arise, these 262 097 RCs will be in immediate danger with minimum chances of escaping to safety. This situation could arise even if the British army was present, although attacks would probably be reduced in scale.

"Except for West Belfast and the Glens area of Antrim, the minority population would be able to offer only the minimum of resistance if attacked."

"Most of the minority is isolated with no avenues of escape and little chance of survival in a doomsday situation. Except for the Belfast area food has not been stocked and there has been no planning or preparations for a crisis situation," the study said.

It suggested the possibility of getting relief supplies into some beleaguered areas by air and evacuating some parts by sea.

It said United Nations involvement would probably not take place until after widespread violence had subsided and "the sides, tacitly at least, ask for some outside force to keep them apart".

Power-sharing self-rule was restored in Belfast earlier this year after a landmark deal between the Republican Sinn Fein and the Protestant-backed Democratic Unionists.

AFP