He was born in Enniskillen, County Fermanagh, Ireland, the eldest child of Professor Nicholas Whistler Colahan (1853-1930) and Elisabeth Quinn of Limerick (b.c.1866). His family moved to Galway, and he grew up there.
After completing his secondary education at St Joseph's College, Galway (The 'Bish'), he enrolled at University College Dublin in 1900, did an Arts degree and then studied medicine. He transferred to University College Galway and graduated in 1913. He was a member of the college Literary and Debating Society and participated in drama.
He began his medical career in the County Infirmary in Galway, and then moved to Holles Street. He joined the Royal Army Medical Corps, and was badly affected by mustard gas in India. After the war he settled in Leicester, where he spent the rest of his career as a neurological specialist.
Colahan was also a composer of popular songs. His most famous work is "Galway Bay", which, popularised by Bing Crosby, was the biggest selling record of all time at one stage. Theories abound as to where the song was written or where it was first heard. Some say it was in the home of Dr Morris at 1 Montpelier Terrace, while others believe it was in The Vicars Croft on Taylor's Hill, from where one could see Galway Bay.
Other songs written by Colahan included Maccushla Mine, Asthoreen Bawn, Until God's Day, The Kylemore Pass and The Claddagh Ring.
- O'Madáin: History of the O'Maddens of Hy-Many, Gerard Madden, 2004. ISBN 0-9529511-7-7.
- The Colahans - A Remarkable Galway Family, Diarmuid Ó Cearbhaill, Journal of the Galway Archaeological and Historical Society, volume 54, 2002, pp. 121–140.
- "Mayor spearheading campaign to recognise 'Galway Bay' composer", Galway Independent, 21 February 2006