This book provides the first comprehensive biographical account of the Third Earl of Rosse. A remarkable man, he was a resident landlord and a key member of the Protestant ascendancy in nineteenth-century Ireland before, during and after the devastating Irish famine of the 1840s. He has received lasting fame as the builder of Birr Castle in County Offaly, where he constructed enormous reflecting telescopes which allowed the recognition for the first time of the spiral shape of some galaxies. As well as giving a comprehensive account of the engineering genius the astronomical achievements of the Earl and his assistants, the book reveals new details about his other key roles, not only in the local community at Birr, but in the Royal Society, of which he was president from 1848-54, in the British Association for the Advancement of Science, and in various political and administrative structures in Ireland. This volume also sheds new light on the family life of the Earl and Countess, whose children included Sir Charles Parsons, the inventor of the steam turbine engine, which revolutionised electricity generation and marine transport. The Countess' own pioneering achievements in photography are detailed here alongside reproductions of her work. With chapters from an international selection of distinguished contributors, this book will interest a wide readership, particularly historians of nineteenth-century British and Irish science, astronomy and photography, and those studying nineteenth-century Irish history.