Danish astronomer, in Ireland from 1874. He compiled three catalogues which together described more than 13,000 nebulae and star clusters; these achieved international recognition as standard reference material. He also wrote a biography of Danish astronomer Tycho Brahe 1890.
Dreyer was born and educated in Copenhagen. In 1874 he was appointed assistant at Lord Rosse's Observatory at Birr Castle in Parsonstown, Ireland. Four years later he took up a similar post at Dunsink Observatory at the University of Dublin, and he was director of the Armagh Observatory 1882-1916.
In 1877 and 1886, Dreyer presented data on new nebulae and corrections to the original catalogue on nebulae and star clusters compiled by English astronomer John Herschel. The Royal Astronomical Society then invited him to compile a comprehensive new catalogue, which Dreyer published 1888 with supplementary indexes 1895 and 1908. His other writings included a history of astronomy.