Shielded by the tighest security screen in Ulster's history, Queen Elizabeth greeted some of her less turbulent subjects at a garden party today, decorated others in a surburban castle and flew back to her yacht, anchored outside Belfast, for a quiet evening.

She never saw the other face of this grim city, a clash between youths hurling bricks and soldiers firing plastic cylinders. The scuffle climaxed a march by supporters of the outlawed Irish Republican Army that was halted by the military at the border of a Catholic enclave half a mile short of its objective, the city hall.

Only minor injuries were reported from his and other incidents, and only seven persons were arrested. By Ulster standards, this was a peaceful day.

British authorities could claim tonight that they had made their political point. The queen could and did come to Ulster, her first visit in 11 years. Moreover, she passes at least at first day of her two-day stay here almost free of unpleasantness.

But the extraordinary security measures, that involved 15,000 troops and 18,000 police, was a potent reminder that she reigns over a province torn by strife. In other parts of her kingdom, she celebrates her silver jubilee by walking through streets to greet throngs of cheering citizens. Here she must travel through heavily guarded corridors to remote places to meet carefully chosen guests.

The queen, Prince Philip and their two young sons, Andrew and Edward, sailed into Belfast Lough this morning aboard the royal yacht, Britannia. They were accompanied by a guided-missile destroyer. Frogmen reportedly had searched the waters for mines and at least one submarine was said to be in the area.

The queen was flown by a red-white-and-blue helicopter from the destroyer to Hillsborough Castle, a yellow stone pile 14 miles from the heart of burnt-out, bombed-out, blackened Belfast. The castle is a political anachronism. In the days when Ulster had a large measure of self-government and was run by its Protestant majority, the queen's governor lived in Hillsborough. It has been empty for five years, ever since London gave up on local rule here and began running the province directly.

The queen, in an emerald-green silk dress, spent her morning inside the castle, distributing knighthoods and other honors awarded to Ulster people for her silver jubilee.

Prince Philip, at the controls of his helicopter, flew off to Harland and Wolff, the heavily subsidized shipyard in East Belfast and the province's biggest employer. There he was greeted by 8,000 cheering workers. This was no surprise, as more than 90 per cent of the work force is a Protestant monopoly and they owe their jobs to their connection with the crown.

Back at the castle, 2,500 businessmen, clergymen, professionals, their wives and other members of Ulster's middle class gathered expectantly on the lush green lawn. The province's Catholic political leaders had also been invited, but even the most moderate, such as Gerry Fitt, thought politically prudent to stay [WORD ILLEGIBLE] Fitt said the food would be too [WORD ILLEGIBLE] for his stomach.

Few of Ulster's Catholic [WORD ILLEGIBLE] support the FRA's terror campaign unite the province by force with [WORD ILLEGIBLE] predominantly Catholic Republic Ireland to the South. But [WORD ILLEGIBLE] the monarch with the [WORD ILLEGIBLE] ascendancy that had been such [WORD ILLEGIBLE] burden.

Queen Elizabeth and Prince [WORD ILLEGIBLE] walked through lanes of [WORD ILLEGIBLE] applauding guests on the castle [WORD ILLEGIBLE] They paused from time to time to change a few words. The ladies in [WORD ILLEGIBLE] colored straw hats and [WORD ILLEGIBLE] printed frocks could have been [WORD ILLEGIBLE] Buckingham Palace garden [WORD ILLEGIBLE] There were even olive [WORD ILLEGIBLE] with watercress sandwiches, tea [WORD ILLEGIBLE] buns. A police band played [WORD ILLEGIBLE] tunes.

While the garden party was in [WORD ILLEGIBLE] the IRA organized about [WORD ILLEGIBLE] its followers in the Catholic [WORD ILLEGIBLE] Andersonstown. With a hundred women in front carrying black flags "to mark our dead," said one - the procession marched down the main stem of Belfast's Catholicism, the Falls Road.

At the bottom, they were met by soldiers wearing plastic visors and carrying plastic shields, other troops with automatic rifles in armored cars, and a wire fence. A voice over a loud-speaker told the procession their march was illegal and asked the crowd to disperse.

When it didn't the soldiers and their vehicles began moving the crowd back. This, as always was the signal for dozens of youths to throw bricks, concrete and bottles at the troops, who took it for a while, then [WORD ILLEGIBLE] "baton rounds" - brown plastic cylinders three inches long.

This drove everyone back to the [WORD ILLEGIBLE] Flats, a grim seven story Catholic housing complex set in rubble [WORD ILLEGIBLE] vacants lots. There the skirmishes continued for several hours. Much the same scene on a smaller scale [WORD ILLEGIBLE] place in Londonderry, although no arrests were made there.