Born in Dublin 15 March 1828, he was the second son of Dr. John McDonnell, from a Hebridean background.
in 1850, obtained the Royal College of Surgeons license in Ireland on 22 February 1851, and was admitted a fellow on 24 August 1853.
He volunteered as a civil surgeon to serve in the general hospital in the camp before Sebastopol, where he remained until the end of the siege.
McDonnell stoutly maintained that the medical officer should exercise full discretion in such matters.
For some years, he was an examiner at the Royal College of Surgeons of Ireland, a body of which he was elected president in 1877.
In 1885 he was elected president of the Academy of Medicine in Ireland, an honourable position he filled for three years.
He died suddenly at his house in Merrion Square, Dublin, on Monday, 6 May 1889, supposedly of rupture of an aneurysm.