Kiernan was born in Mountbellew, County Galway in 1837; his father being a retired British navy surgeon. Kiernan attended Trinity College, Dublin, before emigrating to the USA about 1854. On the outbreak of the civil war in 1861 he joined the 69th New York Infantry Regiment as Assistant Surgeon. He served as such at Bull Run and in the same capacity to the 6th Missouri Infantry Regiment at Pea Ridge.