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. From 1864 to 1883 he was Locomotive, Carriage and Wagon Superintendent of the
Great Southern and Western Railway of Ireland at
Inchicore. McDonnell reformed the GS&WR workshop practices and improved their speed and efficiency. He also introduced practices then in use at
Crewe Works and standardised parts between locomotive classes. McDonnell was noted for his ability to recognise, and employ, men of talent. By doing this, he initiated the "Inchicore school" of locomotive engineers, who would include
Sir John Aspinall,
H.A. Ivatt, and
R.E.L. Maunsell. His first locomotive design for the NER, the
'38' class 4-4-0, incorporated a number of unpopular features; and some popular features of Fletcher's engines were omitted. Worse, although the new locomotives were larger than Fletcher's most recent designs, they were no more capable. His only other design for the NER, the
'59' class 0-6-0, were also unpopular; in this case, the engines were of similar size to Fletcher's most recent 0-6-0, but were less powerful. As a consequence, McDonnell resigned from the NER in September 1884, receiving a year's salary as
severance pay. McDonnell was not replaced immediately, the NER's locomotive department then being managed by a committee chaired by
Henry Tennant. ==Locomotive designs==