Laurence Ross, younger brother of Sir William Parsons, 1st Baronet, settled in Birr, King's County, which became known as Parsonstown, and was the ancestor of another branch of the family. His descendant and namesake Laurence Parsons was created a Baronet, of Birr Castle in King's County, in the Baronetage of Ireland in 1677. He was attainted by King James II's Parliament in 1689 and sentenced to death. The sentence was never carried out, however. His great-grandson, the third Baronet, and great-great-grandson, the fourth Baronet, both represented King's County in the Irish House of Commons.
The latter's half-brother Lawrence Harman Parsons was in 1792 raised to the Peerage of Ireland as Baron Oxmantown, with remainder to his nephew Sir Lawrence Parsons, 5th Baronet, who had succeeded his father the fourth Baronet in 1791. In 1795 he was made Viscount Oxmantown in the Peerage of Ireland, with normal remainder to the heirs male of his body, and in 1806 he was even further honoured when he was created Earl of Rosse in the Peerage of Ireland, with similar remainder as for the barony. Lord Rosse sat from 1800 to 1807 as one of the original Irish Representative Peers in the British House of Lords. On his death in 1807 the viscountcy became extinct while he was succeeded in the barony and earldom according to the special remainders by his aforementioned nephew, the second Earl. He represented King's County in the British House of Commons and sat in the House of Lords as an Irish Representative Peer between 1809 and 1841.
His son, the third Earl, was a well-known astronomer and is famous for his construction of the giant telescope the Leviathan of Parsonstown at his seat Birr Castle. Lord Rosse also sat as Member of Parliament for King's County, was an Irish Representative Peer from 1845 to 1867, Lord Lieutenant of King's County from 1831 to 1867 and President of the Royal Society from 1849 to 1854. He was succeeded by his son, the fourth Earl. He was an Irish Representative Peer between 1868 and 1908 and Lord Lieutenant of King's County between 1892 and 1908. His son, the fifth Earl, was an Irish Representative Peer from 1911 to 1918 and Lord-Lieutenant of King's County from 1908 to 1918. Lord Rosse fought in the First World War and died from wounds received in action in 1918. As of 2007 the titles are held by his grandson, the seventh Earl, who succeeded his father in 1979.