Born into an Irish Protestant family in Portobello, Dublin, Shields started acting in the Abbey Theatre when he was 17 years old. He was the younger brother of Oscar-winning actor Barry Fitzgerald. They were the sons of Adolphus Shields, who "was well-known in Dublin as a labor organizer" although the 1901 census listed his occupation as "press reader," and Fanny Sophia Shields.
Shields returned to the Abbey Theatre and had a varied career there from 1914 to 1939 as actor, assistant Director, Director and stage manager. He appeared in many productions ("more than 300 roles in 350 plays) while he was there, three of the productions he appeared in were by Irish Playwright Teresa Deevy 'The Reapers' 'Temporal Powers' and 'Katie Roche'. Three times he brought the Abbey Company to the United States.
An Irish nationalist, Shields fought in the Easter Rising of 1916. He was captured and held for six months in the Frongoch internment camp in Frongoch, Wales. His obituary in The Times of San Mateo, California, reported, "... upon his release he was decorated by the Republic of Eire."
Married to Bazie Magee in 1920. Son Adam is born in 1927. Married Aideen O'Connor in 1943. Daughter Christine is born in 1946. Aideen died in 1950. Married Laurie Bailey in 1955.
In 1936, John Ford brought him to the United States to act in a film version of The Plough and the Stars. Some of his memorable roles were in Ford films. Shields portrayed the Reverend Playfair in Ford's The Quiet Man, opposite John Wayne, Maureen O'Hara and his brother, Barry Fitzgerald. He played Dr. Laughlin in She Wore a Yellow Ribbon with Wayne and Joanne Dru, and appeared yet again with Wayne and Barry Fitzgerald in Ford's Long Voyage Home. His other films include: Little Nellie Kelly, The Keys of the Kingdom, The Fabulous Dorseys, Gallant Journey, The Shocking Miss Pilgrim, Drums Along the Mohawk, Lady Godiva, National Velvet and The River. He also made television appearances including a 1958 role on Perry Mason as Dr. George Barnes in "The Case of the Screaming Woman."