One of Ireland's finest realist painters, Patrick Hennessy was born in Cork and raised in Scotland by his mother and stepfather along with his brother and sisters. He won a scholarship to the Dundee College of Art, meeting there his life-long friend and companion Henry Robertson Craig. A further scholarship enabled him to study in Paris and Rome.

He returned to Ireland at the outbreak of the second world war, moving alternately being Dublin and Cork. In 1941 he first exhibited at the Royal Hibernian Academy, having already exhibited a still-life and a self-portrait at the Royal Scottish Academy two years earlier.

Over the next thirty years he showed almost one hundred works at the RHA, ranging between interiors and still-lives, portraits, and landscapes. In all of his compositions he employed a distinctive realist style.

In his later years he traveled frequently in Europe and North Africa, often spending his winters in Morocco. A self-portrait hangs in the National Gallery of Ireland, whilst the Crawford Municipal Art Gallery in Cork has five works by him and another four paintings of his can be seen at the Irish Museum of Modern Art in Dublin.