Eileen F. Mahoney, 61, a retired employe of the Army Transportation Corps and a historian of Loudoun County, died March 25 at George Washington University Hospital. She had emphysema.

A resident of Washington for many years, she had returned to the family farm near Aldie, Va., after her retirement in 1970.

Mrs. Mahoney was born at Oatlands near Aldie. She came to Washington as a girl and graduated from Holy Cross Academy and Georgetown Visitation Convent.

In 1939, she was appointed assistant to the court clerk in the criminal division of the old Municipal Court here. During World War II, she worked for the Army Corps of Engineers, transferring later to the Transportation Corps. She did secretarial work and earned several awards.

Mrs. Mahoney was considered an authority on the life of President James Monroe. Her great-grandfather had been brought here from Ireland by Monroe to manage his country estate, historic Oak Hill, near Aldie. She had a collection of diaries and other historic memorabilia.

She is survived by her husband of 40 years, Bernard j. Mahoney Sr., of Aldie; a daughter, Eileen Kowalski, of Potomac; a son, Bernard J. III, her mother, Nellie T. Hogan, and two brothers, William T. and James J. Hogan, all of Aldie, and six grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to the American Lung Association in Fairfax.