The present line of the Earls of Rosse (of the 2nd creation) is descended from Sir Laurence Parsons, one of four sons of James Parsons and Catherine Fenton of Diseworth Grange, Leicestershire, who had moved to Ireland by the late 16th century. The elder brother, William, was the ancestor of the Earls of Rosse of the 1st creation but the line died out in 1764. The younger brother, Sir Laurence lived in Myrtle Grove, Youghal, Co. Cork where he held several Munster-based government positions. He was knighted in 1620, the same year that he moved to Offaly, having exchanged his interest in a property at Leiter Lugna near Cadamstown with Sir Robert Meredith for the latter’s 1000 acres at Birr. In 1677, his descendent, Sir Laurence Parsons was created baronet, and successive generations of the Parsons Baronets have lived at Birr Castle since this time. The earldom of Rosse was inherited by Sir Laurence Parsons, 5th baronet, from his uncle Laurence Harman Parsons, 1st Earl of Rosse, of County Longford, who died in 1807 without male issue.
Sir Laurence Parsons, 2nd Earl of Rosse (1758-1841), was an Irish peer, agitator against the Act of Union, an Irish parliamentarian and later joint postmaster-general of the Irish post office. His son, Sir William Parsons, 3rd Earl of Rosse (1800-1867), was an astronomer and in 1845 built the ‘Leviathan of Parsonstown’, the world’s largest telescope until the early twentieth century. Sir Laurence Parsons, 4th Earl of Rosse (1840-1908) was also an astronomer and a keen photographer like his mother, Mary Rosse. Sir William Edward Parsons, 5th Earl of Rosse (1873-1918), was a solider in the Irish Guards. He fought in the First World War and died in 1918 of injuries received in action two years previously. His son, Sir (Laurence) Michael Parsons, 6th Earl of Rosse (1906-1979) was heavily involved in Irish cultural affairs and a keen dendrologist. The present Earl of Rosse, and 10th baronet, Sir (William) Brendan Parsons was an officer in the Irish Guards from 1955–57 and worked for the United Nations from 1963-80. He lives at Birr Castle and has overseen the creation of the Historic Science Centre celebrating the scientific legacy of the Parsons family, and, with the assistance of Dr A. P. W. Malcomson of PRONI, has gathered together the archives of the Parsons family, now published as The Calendar of the Rosse Papers.