Robert Stewart, 1st Marquess of Londonderry PC (Ire) (1739–1821), was a County Down landowner, Irish Volunteer, and member of the parliament who, exceptionally for an Ulster Scot and Presbyterian, rose within the ranks of Ireland's "Anglican Ascendancy."

His success was fuelled by wealth acquired through judicious marriages, and by the advancing political career of his son, Viscount Castlereagh (an architect of the Acts of Union, and British Foreign Secretary).

[22] This political triumph over the interests of an Ascendancy family which had hitherto returned both county members to the Irish House of Commons formed the prelude of a long period of rivalry.

Robert Stewart's initial success was largely due to popular sympathy with John Wilkes and the discontented American colonists, and to the growing feelings in favour of constitutional and parliamentary reform which found expression in the Volunteer movement.

A dinner at which they entertained in Belfast was marked by toasts "liberal in quality as in quantity", including to "The memory of John Hampden" (who had led parliamentary opposition to Charles I), and to "All those who would rather die in jack-boots than live in wooden shoes".

Stewart participated in the Irish Volunteers, the self-armed militia ostensibly formed to maintain order and defend Ireland while the Crown and its forces were distracted by the American War.

The digested bill, presented by Henry Flood, which would have abolished the proprietary boroughs (with which their Ascendancy rivals, but not the Stewarts, were endowed) and extended the vote to a broader class of Protestant freeholders was rejected.

[45] Although he believed that the demands of Dissenters for greater representation should have been met so as to dissuade them from pushing Catholic claims along with their own,[46] Stewart joined his friend, president of the convention, the Earl of Charlemont in urging the Volunteers to receive their rebuff quietly.

[56] His eldest son, now Viscount Castlereagh, also quietly converted to Anglicanism and was appointed Chief Secretary for Ireland to serve under Lady Frances's brother, Earl Camden, the Lord Lieutenant.

[57] Banking on these new establishment connections, and alarmed by the evident disaffection of their tenantry, Stewart reached an accommodation with the Hills: the families in future would divide the two county seats in Down, each returning a nominee to the parliament in Dublin unopposed.

[62] In August, the wife of the local Presbyterian minister, James Porter, appeared at the house with her seven children where they overwhelmed Lady Londonderry and young sister, then dying of tuberculosis, with a plea for his life.

One of the children was later to recount that when Londonderry discovered his wife composing a letter to General Nugent, he insisted she add a postscript: "L does not allow me to interfere in Mr Porter's case.

Henry Montgomery of Killead, County Antrim, would later describe the circumstances of Porter's execution as being of "extreme cruelty towards both himself and his family, which were altogether unnecessary for any purpose of public example".

[71] Porter's offence may have been his popular satire of the local landed interest, Billy Bluff, in which the master of Mount Stewart is clearly recognisable as the inarticulate tyrant "Lord Mountmumble".

[68] It may also be that Londonderry believed that Porter, who had been close to the family (their election agent and a frequent visitor to the Mount Stewart),[73] had been a source of his wife's wayward, and potentially compromising, political sympathies.

Lady Frances is rumoured to have continued to send privately for Porter's offending paper, the Northern Star,[72] and in correspondence with Jane Greg (reputedly "head of the [United Irish] Female Societies" in Belfast)[74] made bold to identify herself as a "republican countess".