Colgan, Nathaniel (1851–1919), botanist, marine biologist, and traveller, was born 28 May 1851 in Dublin. The identity of his parents is not known with certainty, though they may have been Nathaniel Watson Colgan and Letitia Phair, who married in Dublin in 1846. Colgan was educated at the incorporated school, Aungier Street. In 1871 he obtained by examination a clerkship in the Dublin metropolitan police court, and worked there until he retired in 1916. In 1873 he was involved in the production of a magazine, Varieties, to which he contributed a number of papers and which he later edited. He also contributed to the Irish Monthly, Tinsley's Magazine and Hibernia, generally writing about the many journeys to continental Europe and north Africa that he undertook in the summer months from 1875 onwards. He visited France, Italy, Switzerland, Spain and Morocco as well as many parts of Ireland.
From 1880 botanical notes began to appear in his articles, although his ‘Plant hunting in the Dublin mountains’ in the Irish Monthly of 1880 was a skit. In 1884 his discovery of sawwort, a very rare plant, in eastern Ireland led to his meeting A. G. More (qv), and his first scientific publication, in the Journal of Botany. This increased Colgan's interest in botany and led to a number of botanical expeditions, one of which to Kerry, in the company of R. W. Scully (qv), may have inspired him to produce his Flora of the County Dublin, which appeared in 1904. The work was a model of its kind, though it had been delayed because, by More's will, Colgan had been given responsibility for publishing the second edition of More's Cybele Hibernica; he collaborated on this major enterprise with Scully, and the volume appeared in 1898. Colgan was responsible for the sections on problems of nomenclature, old records, examination of soil types and Irish names of plants. In 1900 he moved to Sandycove, Co. Dublin; living beside the sea led him to study marine fauna, particularly mollusca and tunicates (sea squirts). He dredged for specimens with the Dublin Marine Biological Association, and in the RIA's Clare Island survey he undertook the work on mollusca and enquired into the local names of plants and animals. He was a member of the RIA from 1894 and joined the Dublin Naturalists’ Field Club on its foundation in 1886. Though he was a member of a number of organisations, he suffered greatly from shyness. He wrote many papers in scientific journals, on Irish naturalists abroad, on various aspects of folklore including witchcraft, and on plant movement. In the 1890s he tried to establish the botanical identity of the plant known as the shamrock, so closely associated with Ireland in popular tradition; his requests to correspondents in different counties for plants of the ‘true shamrock’ produced a number of distinct species. His 1896 monograph on the history of the shamrock in Ireland is still the definitive account, and probably his best-known work. He died 2 October 1919 in Dublin; he seems to have been unmarried.