A former Internal Revenue Service agent was sentenced yesterday to 90 days in jail for failure to report $675 in income in 1977.

The former agent, Ronald Gilmartin, 42, of 2416 Arlington Blvd., could have received five years in prison and a $10,000 fine.

U.S. District Judge Albert V. Bryan Jr. told Gilmartin that although the amount of income that went unreported was small, "IRS agents have a greater obligation" than others to report their incomes accurately.

Gilmartin, the divorced father of five children, was indicted on Dec. 3, 1979, on three other tax evasion charges. One of those charges involved his failure to report $400 in income, and the other two technical counts involved failure to report that he was receiving fees for preparing income tax forms while he was employed by the IRS.

Those other counts were dropped after Gilmartin pleaded guilty to falsifying his 1977 tax records. In that year, his actual income was $31,224.02 although he reported that it was only $30,549.02, a difference of $675, court papers state.

Defense attorney John J. Geraghty told Bryan that Gilmartin had received the unreported funds from a young man he "regarded as a son" in repayment of a loan.

Bryan also placed Gilmartin on two years probation.