Harriette Mary Lombard Sheehy was born on 23 June 1871 in Dublin, Ireland.[3] She was the daughter of Charlotte (née Lombard) and Edward Bryan Sheehy.[1] In 1892, she married James Walter Hennessy.[1][4] The couple had two children: Christopher James (born 1893), and Olive Mary Hope (born 1894).[1] James Walter Hennessy died in 1896.[1]
Hennessy gained her M.D. degree in Brussels,[5] and was licensed to practise by the Royal College of Physicians in the same year.[3][6] By this time, medical schools in Ireland were open to women, and she was licensed to practise as both a physician and a surgeon.[3] She opened her own medical practice in Fitzwilliam Square, Dublin.[3]
Hennessy was involved with the Irish Women's Suffrage and Local Government Association, including by holding drawing room meetings at her Dublin home.[7] She was also associated with the Irish Women's Progressive Union, for whom she spoke.[8]
Hennessy acted as medical advisor to the Eleventh Edition of the Encyclopædia Britannica.[3] She was given charge of sub-editing entries on medical topics, as well as contributing a number of her own.[3][9] These appeared under her initials, "H. L. H.".[9]
Hennessy died on 13 April 1913 in Killavullen, Ireland.