WHITE PLAINS, N.Y. (AP) _ A man whose two previous arson-murder convictions in a 1974 discotheque fire were overturned on appeal pleaded guilty today to second-degree manslaughter in the blaze, which killed 24 people.

Peter Leonard, 34, of Greenwich, Conn., entered the plea to 24 counts of manslaughter before Westchester County Court Judge Peter Rosato.

The district attorney's office agreed to the reduced charges, which carry a maximum term of 15 years in jail.

Leonard's laywer, Stephen Pittari, told Rosato that Leonard already has served just under 12 years in jail and that when time off for good behavior is figured in, he would be eligible for release at once.

Fire swept through the popular disco, Gulliver's, on the line between Greenwich, Conn., and Port Chester, N.Y., on June 24, 1974, a crowded Saturday night. All the dead and the 27 people who were injured were under age 35.

Leonard was arrested a few weeks later after telling police he had set the fire to cover up a burglary in a bowling alley adjoining the night club.

He pleaded guilty to 24 murders in June 1975 in a deal for a sentence of 15 years to life. He could have gotten 25 years to life.

The conviction was overturned by the Appellate Divison in a 1977 ruling that Leonard's confession had been coerced by police.

At a trial in 1978, a Westchester County Court jury found Leonard guilty of 24 counts of murder and also convicted him of arson, burglary and petty larceny.

That conviction was overturned last December, by the Appellate Division, on grounds that Leonard had made incriminating statements to police without a lawyer present, although police knew that Leonard had a lawyer for an unrelated larceny charge.