Flora of the County of Dublin. Flowering Plants, Higher Cryptogams, and Characeae. With a Map. Dublin: Hodges, Figgis, 1904. First Edition. Pp lxx, 324, uncut, top edge in gilt  Original green buckram, title in gilt on spine, covers ruled in blind. A fine bright copy. [With] A Supplement to Colgan’s Flora of the County of Dublin. Published by the Stationary Office, 1961. Pp 95. Frontispiece map. Fine in publisher’s wrappers.

Regarded as one of the most important contributions to Irish botany, Colgan’s methodology set a high bar for county floras.

In his preface, he outlines strict personal conditions for inclusion: he insisted on seeing every species growing with his own eyes in at least one Dublin station, and he personally recorded their vertical ranges, flowering seasons, and soil affinities . He also sought to document popular plant names from local country people, adding an ethnographic dimension to the botanical record.

Nathaniel Colgan (1851–1919) was a keen amateur naturalist, also known for his definitive 1896 monograph on the history of the shamrock . In the closing lines of his preface he expresses the hope that rising Dublin botanists would render his work obsolete through their own research—a mark of his scientific spirit.