Oxmantown’s father had been a prominent opponent of the Union in the Irish House, where he had represented Dublin University, 1782-91, when he succeeded to the family’s seat for King’s County, but on taking his place at Westminster he became a supporter of the Addington and Pitt ministries and was rewarded with office in the Irish treasury in 1805. In 1807 he succeeded to his uncle’s earldom of Rosse and in 1809 was elected an Irish representative peer and appointed Irish joint-postmaster general, a non-resident sinecure worth £1,500 a year with a ‘great deal of patronage attached to it’, which he held under successive governments until his resignation on the fall of the Wellington administration.
At the 1826 general election he offered again as a supporter of Catholic emancipation and was returned unopposed.
At the 1830 general election Oxmantown offered again, professing ‘perfect independence’ from party, and was returned unopposed.
At the 1832 general election Oxmantown offered again and was narrowly returned in second place as a Liberal. He retired at the 1834 dissolution in order to concentrate on his significant experiments with large astronomical telescopes, which he had begun in 1827. In the early 1840s, after succeeding his father in the peerage, he constructed a ‘monster’ telescope at his Birr Castle residence at an estimated cost of £20,000. Known as the ‘Leviathan of Parsonstown’, it was the largest in existence for over six decades. He sat in the Lords as a ‘moderate Conservative’ representative peer from 1845.
