A. Dev O'Neill, 56, the official photographer for the U.S. House of Representatives from 1956 to 1978, died at Sibley Memorial Hospital Monday following a stroke.

Mr. O'Neill was born in Cork, Ireland. His family moved to the United States when he was a youngster and he grew up in Ohio and Durham, N.C. He attended Duke University and the University of North Carolina. During World War II, he was a pilot in the Royal Canadian Air Force and then in the U.S. Army Air Corps. He was a colonel in the Air Force Reserves.

He first came to Washington in 1952 as a reporter and photographer for a North Carolina news service. In 1956, the late Sam Rayburn, then speaker of the House, hired him as a photographer for the house. When he retired last year, Mr. O'Neill was head of the office of photography for the House of Representatives.

At the time of his death, Mr. O'Neill was writing a book about his experiences photographing well known persons over the years.

Survivors include his wife, Lucy, of the home in Washington, and three children, Thomas and Laura O'Neill and Peggy Thatcher, all of Washington.

The family suggests that expressions of sympathy be in the form of contributions to the Sidwell Friends School scholarship fund in Washington. CAPTION: Picture, A. DEV O'NEILL