 He was educated at Trinity College, Dublin, where he graduated with an honours BA in mathematics (1851).He brought order and standardisation to the workshops and locomotive designs of the Great Southern and Western Railway of Ireland, and was later employed to do the same for the North Eastern Railway in England, although resistance to his changes meant little progress was made before he left.McDonnell was apprenticed at Newall and Gordon in Westminster before working as an engineer at the Newport, Abergavenny and Hereford Railway – later to become part of the Great Western Railway.