Britain tries to deal with thorny Ulster question

Among the results of one of the most lethal bomb blasts in the history of Northern Ireland:

* Near-total condemnation across Britain of the tactics of terror used by Irish republican forces.

* Calls for increased security by Protestants in Ulster, some of whom claim that the British policy of gradually turning over more security work to the Ulster police and away from the Army should now be reversed.

* New determination by the Thatcher govenment in London to defy the terrorists by keeping British troops in place.

* Intense pressure from Tory and Labour members of Parliament alike on the leader of the Greater London Council (GLC), Ken Livingstone, to cancel an invitation to two Sinn Fein members of the new Ulster assembly to visit London Dec. 14. (The Provisional Sinn Fein Party, which openly advocates violence to drive out British troops and create a ''Democratic Socialist Republic of Ireland ,'' now has five seats in the new 78-seat assembly in Belfast.)

* A dilemma for London police. It is their responsibility, under a law for the prevention of terrorism, to recommend to the Home Secretary William Whitelaw , whether the visit should be stopped.

At this writing, the police had not acted but Prime Minister Margaret Thatcher herself and other senior government officials made it clear that they thought Mr. Livingstone should withdraw the invitation.

But Mr. Livingstone has said the visit would go ahead as scheduled. ''It is because of the bombing, because this has gone on for 13 years with one horrifying bombing or murder after another, that I believe it is time to start to try and get some dialogue going,'' he said Dec. 7.

Many here suspect that one Livingstone motive was to embarrass the government and Labour leader Michael Foot. As a prominent member of Labour's far left wing, Mr. Livingstone makes no secret of his own view that British troops must leave and Ireland be united.

Mr. Foot quickly moved to condemn IRA terror, but in a letter before the bomb explosion, failed to demand that the visit be canceled.

The bomb changed matters. Coming after Republican bomb attacks on the Household Cavalry in London, and after a huge find of explosives by police in the south, the visit was loudly condemned by Roman Catholic clergy and Tories and Labourites alike.

The bomb and its political fallout still dominated British newspaper frontpages, television screens, and radio broadcasts two days after it exploded.

It came at a time when hopes had been slowly rising that the Protestant and Roman Catholic communities in Northern Ireland might be more ready to listen to the pleas for peace being heard on all sides.

The very community in which the bomb detonated - Ballykelly, not far from the Catholic city of Londonderry, was considered a model of sectarian cooperation. Residents of Ballykelly claimed that the terrorists were encouraged to choose their town as a target because the Army recently dismantled permanent roadblocks.

The immediate effect of the bomb, which killed 16 people and wounded 66 late Dec. 6, was to heighten Protestant fears and strengthen Mrs. Thatcher's policy of using troops to maintain security.

The response in London was not to send more troops but rather to confine the 10,000 off-duty British soldiers to barracks except on well-organized sporting and other recreation outings.

The bomb blast was condemned in the strongest terms by the Prime Minister and by James Prior, the secretary of state for Northern Ireland. Mr. Prior called it ''massacre without mercy.''

Responsibility has been claimed by the extreme left-wing Irish National Liberation Army.