Born in Dublin, Walsh read law at the National University of Dublin and studied acting at the Abbey Theatre School. Signed by Rank, he enjoyed in a brief period of stardom, appearing opposite Margaret Lockwood before continuing his movie career in second features from the 1950s. He became best-known for his role as Richard the Lionheart in a swashbuckling 60s TV series.
The first of his three wives was actress Hazel Court (1949-63). He was the father of actress Elisabeth Dermot-Walsh (1974-).