Fairfax County has tentatively agreed to pay $2.75 million and hire 150 blacks and women to settle a long-running dispute with the U.S. Justice Department over county hiring practices.

The money would be distributed to a still-undetermined number of blacks and women who applied unsuccessfully for county jobs after 1975. After a federal judge in Alexandria ruled last March that Fairfax was guilty of discrimination, 1,925 such applicants have come forward--1,850 of them before a deadline for submitting claims expired and 75 after the deadline. It is unclear how many of the latter group will be eligible for compensation.

The tentative settlement marks a reversal by county officials, who had vigorously contested the Justice Department lawsuit. John F. Herrity, Republican chairman of the County Board of Supervisors, wrote last year to President Reagan and Attorney General William French Smith urging them to drop the suit on the basis of campaign pledges to end harassment of local officials.

But Fairfax attorneys apparently convinced the board that enough claimants could show evidence of discrimination to make contesting the suit more expensive than settling it. The nine supervisors unanimously approved the outlines of a settlement in a closed meeting last week.

County officials stressed that final negotiations with Justice attorneys remain at a sensitive stage, and supervisors would not comment publicly last week. The settlement, under which Fairfax would admit to no wrongdoing, does not become official until approved by U.S. District Judge Albert V. Bryan Jr., who ruled against the county last March.

A spokesman for the Justice Department, John V. Wilson, also declined to discuss the specifics of the settlement negotiations.

"We're trying," Wilson said, referring to the months-long talks between U.S. and Fairfax attorneys. "We're dealing with another public agency, and this would save time and expense on both sides, for national taxpayers and for county taxpayers."

The Justice Department first sued Fairfax late in 1978, and Judge Bryan ruled in April 1979 that the U.S. had not proved Fairfax had discriminated. The 4th U.S. Circuit Court of Appeals then ordered Bryan to hear the case again, giving more consideration to some of the federal government's statistical evidence.

Last year Bryan ruled that Fairfax had discriminated against blacks in professional, technical and police jobs, and against women in paraprofessional positions. In 1977, 5.6 percent of the county's 7,000 workers were black. That figure has risen to more than 10 percent.

The judge ordered the county to send letters to as many unsuccessful applicants as it could find and advertise in area newspapers to notify others of their possible eligibility for back pay awards. Bryan appointed an Alexandria lawyer as special master to rule on the merits of each claim.

Those hearings are scheduled to begin in April, according to special master Thomas Rawles Jones. If the case is settled, the hearings will not go forward.

Instead, the Justice Department and the county would agree on the sum to be paid to each deserving claimant as part of the settlement. County attorneys said the $2.75 million would be less than the cost of back pay and litigation if the suit goes forward, which they estimated could range from $3.5 million to $17 million, according to officials familiar with the case.

The settlement would also specify which 150 claimants would be hired as jobs become open. The county would not have to create any jobs.

Herrity declined last week to discuss the settlement, but said he would not approve any agreement that imposed a quota system on county hiring practices.