DUBLIN, Ireland (AP) _ Fire swept through a 150-year-old Roman Catholic convent school early Monday and killed six nuns in the top-floor dormitory where they were sleeping. Fifteen other nuns escaped.

Police said all six victims were from Ireland. The six, whose ages ranged from 60 to 83, were retired teachers.

Some younger nuns ran back into the burning, four-story building to try to rescue those who were trapped, ''but they were driven back out, choking from the smoke, and could not see,'' said Sister Patricia Murray, spokeswoman for the Loreto Order which runs the Loreto Secondary School for Girls.

Three dead nuns were found huddled together in a corridor, and the other three were elsewhere in the dormitory, the fire brigade said.

Nightclub owner Derek Walsh said: ''I was passing by in my car when I was waved down by a nun who ran out of the building in her dressing gown.''

He said: ''I know the layout of the building because my daughter goes to school there. Using a torch (flaslight) from my car I tried to get to the trapped nuns but was driven back by black smoke.''

The cause of the fire was not known, said a fire department spokesman who demanded anonymity.

The tragedy shocked the overwhelmingly Catholic republic and brought condolences from President Patrick Hillery, Prime Minister Garret FitzGerald and opposition leader Charles Haughey.

''The fire was well alight when the sisters found out about it,'' said Sister Murray. ''All the sisters who died were moving to the exits. The floor or the stairs must have collapsed.''

A patrolman noticed the fire at 3:30 a.m. and alerted the fire brigade. Then terrified nuns began rushing from the flaming, 19th-century building.

The school overlooks St. Stephen's Green, a large oblong park in the center of Dublin surrounded by government offices, shops and the Shelbourne, one of the city's finest hotels. The fire was brought under control after two hours. Other buildings were not damaged.

The Loreto Secondary School, founded in 1833, has 650 high school girls. It does not take boarders, and the nuns were alone in the building during the fire.

The nuns killed in the fire were identified as Sisters Eucharia Scully, 83, Rosario Flavin, 65, Margaret Keane, 60, Edith Kennedy, 68, Gonzaga Kehoe, 81, and Seraphia Kennedy, 76.