Answer by taking a quote from the following article:

Robert Stewart, 2nd Marquess of Londonderry,  (18 June 1769 - 12 August 1822), usually known as Lord Castlereagh, which is derived from his courtesy title Viscount Castlereagh, (UK:  KAH-s@l-rey), was an Irish/British statesman. As British Foreign Secretary, from 1812 he was central to the management of the coalition that defeated Napoleon and was the principal British diplomat at the Congress of Vienna. Castlereagh was also leader of the British House of Commons in the Liverpool government from 1812 until his suicide in August 1822. Early in his career, as Chief Secretary for Ireland, he was involved in putting down the Irish Rebellion of 1798 and was instrumental in securing the passage of the Irish Act of Union of 1800.

Stewart had recurring health problems throughout his childhood, and his family elected to send him to The Royal School, Armagh, rather than to England for his secondary education. At the encouragement of Charles Pratt, first Earl of Camden, who took a great interest in him and treated him as a grandson by blood, he later attended St. John's College, Cambridge (1786-87), where he applied himself with greater diligence than expected from an aristocrat and obtained first class in his last examinations. He left Cambridge due to an extended illness, and after returning to Ireland did not pursue further formal education.  In 1790, Stewart was elected as a Member of the Irish Parliament for Down in one of the most expensive elections in Irish history. Though for a time he was associated with the Northern Whig Club, he entered the Irish House of Commons as an Independent. He ran on a platform supporting Whig principles of electoral reform and opposing the Irish policies of the British Government. But even from the outset of his career, he was a personal supporter of the Prime Minister, William Pitt. Stewart was a lifelong advocate of Catholic concessions, though his position on the specific issue of Catholic Emancipation varied depending on his assessment of the potential repercussions on other policy priorities.  When war with France forced British Government attention on Ireland as a possible place of French invasion, the Irish Volunteers, seen as a potential source of disaffection, were disbanded by Dublin Castle, and a reorganised Militia was created in 1793. Stewart enrolled as an officer, a matter of course for a young Protestant aristocrat, and served as Lieutenant Colonel under the command of his wife's uncle, Thomas Conolly. Between Stewart's attendance to his militia duties, his pursuit of cultural, family and political interests in London, two trips to the Continent (in 1791, when he visited revolutionary Paris, and 1792), and the courtship of his wife whom he married in 1794, his life during this period was not centred on the activities of the Irish House of Commons, where he was listened to with respect but where he was not yet an important player. He was also beginning to disappoint some of his more radical original supporters in his constituency. As the French Revolution grew more bloody and Ireland more rebellious, Stewart increasingly worried about Ireland's future if the threats from France succeeded in breaking Ireland's links to Britain. He became further inclined to support not only Pitt personally but the British Government, even when he did not approve of a specific line taken in Irish policy.  In 1794, partly as a result of the promotion of Stewart's interests by his Camden connections, he was offered the Government-controlled seat of Tregony in Cornwall, where he was elected to the British House of Commons on a similar platform of reform principles and support for Pitt, on whose side he sat in Westminster. In 1796, he transferred to a seat for the Suffolk constituency of Orford, which was in the interest of his mother's family, the Seymour-Conways (Marquess of Hertford).

What did he do after returning to Ireland?
Stewart was elected as a Member of the Irish Parliament for Down