Dermot Michael (Pat) O'Neill, 80, a retired consultant on security and police procedures for the State Department who had served in a commando unit during World War II, died of pneumonia Aug. 11 at Suburban Hospital.

Mr. O'Neill was born in County Cork, Ireland. When he was still a teen-ager he went to Shanghai, where he became an expert in forms of hand-to-hand combat and served on the international police force. In the late 1930s he was head of security at the British Embassy in Tokyo.

He left Japan for Australia shortly before the Japanese attack on Pearl Harbor. In 1942 he came to the United States and joined the Army, where he was assigned to a joint U.S.-Canadian commando unit. During World War II he served in the Aleutians, North Africa, Italy, southern France and Okinawa.

After the war he became a consultant on police procedures for the State Department and was posted in Florida for most of the time until the mid-1960s, when he retired and moved to Washington. In retirement he had continued consulting part time until the early 1970s at the old International Police Academy here.

His marriage to Mary Frances Hagedorn ended in divorce. He is survived by a daughter, Barbara O'Neill Mero of Bend, Ore.