[1][2][3] Barrett-Hamilton was born in Ahmednagar, India,[4] of Irish parents, who returned and settled at Kilmanock in County Wexford when the boy was three years old.
He was educated at Harrow and Trinity College, Cambridge,[5] spending summer holidays botanising at home under the encouragement of Alexander Goodman More.
[2] From 1887 to 1908 Barrett-Hamilton contributed papers on Wexford plants to the Journal of Botany, British and Foreign and to The Irish Naturalist.
[8] After the war ended in June 1902, he left Cape Town in the SS Dunera in late September 1902, arriving at Southampton early the following month.
[8] In his work as a natural historian, he described a great number of new species of small mammal on the islands around the British Isles, notably the house mice and field mice of St. Kilda which he called Mus muralis and Mus hirtensis,[10] believing that these had evolved in situ having colonised the islands naturally via land or ice-bridges.