Britain's role in Northern Ireland has been seriously and directly challenged by the loyalist ''day of action'' Nov. 23, when thousands of Protestants in Ulster brought business life here to a standstill.
The initial government response has been low-key, but the scale of the protest against the lack of action against the Provisional wing of the illegal Irish Republican Army (IRA) threatens to create the biggest crisis in the province since the attempted all-out work stoppage in 1977.
Prime Minister Margaret Thatcher is specifically challenged on her Anglo-Irish policy. The Rev. Ian Paisley and his supporters see the London-Dublin talks as the prelude to a ''sell-out'' of the Ulster unionists. Mrs. Thatcher insists that this is not so and maintains that closer cooperation will help both governments deal with cross-border problems arising out of the Ulster crisis.
The determination on both sides not to bend now seems to signal a showdown between Mr. Paisley and Mrs. Thatcher. Many people in Northern Ireland fear that the logic of Mr. Paisley's course of action is ultimately an independent Ulster. John Cushnahan, secretary of the moderate Alliance Party, said Nov. 24 ''it would seem that independence is now clearly Ian Paisley's objective.''
3 James Prior, Britain's secretary of state for Northern Ireland is being challenged directly over his handling of the Ulster crisis. Last week he declared that private armies would not be tolerated. Yet on the night of Nov. 23 , some 5,000 members of the so-called ''Third Force'', a Protestant vigilante organization, paraded in Newtownards in the presence of Mr. Paisley who pledged himself to recruit a total of 100,000 men and women ''to destroy the IRA.''
The less militant Official Unionist Party is challenged as well by the new support given to the Paisley protest. More than twice as many Paisleyites turned out to support a Democratic Unionist rally at Belfast City Hall, only minutes after the Official Unionists rally had ended.
The other key issue, apart from the Anglo-Irish Council, is security. Loyalists of all shades, some of whom vary in their opposition to the London-Dublin talks, are agreed not enough is being done by the government to combat the IRA.
In reality the IRA is an extremely formidable enemy. It is organized in small cell structures with the capacity to plan a quick military strike and then melt into the background. A spate of recent murders of part-time soldiers and policemen, most of them Protestants in isolated country areas, has increased loyalist tensions. The crisis was brought to a head by the killing of the Unionist MP, the Rev. Robert Bradford.
Mr. Paisley's ''Third Force'' now has the avowed intent not only to protect Protestants but to ''exterminate'' the IRA. The question now facing Mrs. Thatcher's government is to what extent this is rhetoric or dangerous reality.
Since the death of Mr. Bradford, which precipitated the present crisis, the government response has been relatively low-key. An extra battalion of 600 trooops has been flown in and police leave has been cancelled. Some observers believe that even more troops are needed, if only to reassure a nervous population - including the Roman Catholic community - of a larger and more visible security network.
Security experts say privately that the real job of catching the terrorists is still largely an undercover operation which depends largely on information fed to the security forces.
In recent days leading members of the Roman Catholic Church have spoken out strongly against the IRA. If the IRA continues to lose more grass-roots Catholic support, they could become an easier target for the security forces.
So far Mrs.Thatcher has given no indication that there will be any slackening of momentum on the Anglo-Irish talks. But if the government is serious about decreasing tension, it may well need to consider sending more troops and to find some means to continue the cross-border discussions on a level which will maintain contacts without causing loyalists any further alarm.