After Napoleon's second abdication in 1815, Castlereagh worked with the European courts represented at the Congress of Vienna to frame the territorial, and broadly conservative, continental order that was to hold until mid-century.

[b] The legacy from Robert Cowan, the former Governor of Bombay, allowed for the purchase of extensive properties in north Down including the future family demesne, Mount Stewart, on the shores of Strangford Lough.

At the encouragement of Charles Pratt, first Earl of Camden, who took a great interest in him and treated him as if he had been a grandson by blood, he later attended St. John's College, Cambridge (1786–87),[7] where he applied himself with greater diligence than expected from an aristocrat and excelled in his first-year examinations.

However, news of the Jacobin triumph in Paris, and, following her victory at Valmy, the prospect of France carrying defence of the revolution beyond her frontiers convinced him that "it would not be long before he had to face his own 'Jacobins' in Ireland.

In doing so he was supporting the policy of British Prime Minister William Pitt who determined that Catholic opinion be conciliated in preparation for the impending war with the new, anti-clericalist, French Republic.

[12] While calling for the removal of their remaining civil disabilities, Stewart stopped short of endorsing extension to Catholics of the right to vote on the same forty-shilling freehold terms as Protestants that was provided in the bill.

[4]: 96–100, 107–108  Originating in Belfast among Presbyterian celebrants of the American and French revolutions, the republican conspiracy had spread rapidly in Ulster and, in league with the Catholic Defenders, across the Irish midlands.

In County Down, Castlereagh's father had difficulty in raising a loyalist yeomanry among his tenants and eventually, with all rent withheld, Mount Stewart was placed under armed guard.

[g] At the urging of Camden, Castlereagh assumed many of the onerous duties of the often-absent Chief Secretary for Ireland, who was responsible for day-to-day administration and for asserting the influence of Dublin Castle in the House of Commons.

The judge reportedly broke down in tears as he read the death sentence which the popular journalist Peter Finnerty credited to Castlereagh's insistence on making an example in the face of the growing French fever.

An exception was made in the case of James Porter, executed, again despite the entreaties of Lady Frances, following a court martial before Castlereagh's father, Lord Londonderry.

[23] Porter, who had been his family's Presbyterian minister and, in 1790, his election agent, had become a household name in Ulster as the author of a satire of the county gentry, Billy Bluff, in which Londonderry was serially lampooned as an inarticulate tyrant.

[24][25] In 1799, in furtherance of both his own political vision and Pitt's policies, Castlereagh began lobbying in the Irish and British Parliaments for a union that would incorporate Ireland with Great Britain in a United Kingdom.

In addition to security against the French, Castlereagh saw the principal merit of bringing Ireland directly under the Crown in the Westminster Parliament as a resolution of what ultimately was the key issue for the governance of the country, the Catholic question.

[4]: 127  "Linked with England", he reasoned that "the Protestants, feeling less exposed, would be more confident and liberal", while Catholics, reduced to a minority within the larger kingdom, would lower their expectations and moderate their demands.

[26] During the campaign for the Act of Union, both Castlereagh and Cornwallis had, in good faith, forwarded informal assurances they had received from Pitt's Cabinet to the Irish Catholics that they would be allowed to sit in the new United Kingdom Parliament.

A separate Irish executive in Dublin was retained, but representation, still wholly Protestant, was transferred to Westminster constituted as the Parliament of the United Kingdom of Great Britain and Ireland.

Castlereagh would long be held personally responsible by many Catholics in Ireland for the breach of promise and the British Government's failure to remove their remaining political disabilities.

[5][27] In the new Parliament of the United Kingdom the tensions within the ruling Tories over Catholic emancipation abated, and after obtaining his desired cessation of hostilities with France (the Peace of Amiens), in July 1802 Henry Addington brought Castlereagh into the Cabinet as President of the Board of Control.

[13]: 107  On entering the cabinet he wrote to Addington deploring the role of the Orange Order in fostering sectarian violence and to commend putting "the law rigidly in force against all parties" so that in future wars, "our foreign enemies" would not again find an aggrieved domestic ally.

[30] His efforts to extend a similar scheme to the Catholic clergy met with stiffer resistance: priests would not accept support from the Crown while it continued to deny their communicants political equality.

The Dowager Marchioness of Downshire broke her family's electoral truce with the Stewarts in County Down and in July 1805 forced Castlereagh to defend his (now Westminster) parliamentary seat.

In the contest he also faced the hostility of unrepentant United Irishmen—men like William Drennan who were to engage in what Castlereagh denounced as "a deep laid scheme again to bring the Presbyterian Synod within the ranks of democracy".

[32][33] Despite the prestige of a new cabinet position in London, Castlereagh was defeated in a campaign marked by repeated aspersions on his failure to father a child, and by the taunts of those who, otherwise no friends of the Downshires, reminded him of the principles on which he had stood in 1790.

Having conciliated the Downshires and able to ride the victories of Arthur Wellesley (future Duke of Wellington), the reputed Dubliner he had appointed to command in the Peninsular War, in 1812 Castlereagh avenged his humiliation, and recovered the family seat.

[35] In 1808 Castlereagh had been warned by Dumouriez that the best policy England could adopt with respect to colonies in Spanish America was to relinquish all ideas of military conquest by Arthur Wellesley and instead support the emancipation of the territories.

In what were the "verbal equivalents of the political cartoons of the day",[49] Tom Crib's Memorial to Congress (1818) and "Fables for the Holy Alliance" (1823), Moore savages Castlereagh's pirouetting with Britain's reactionary continental allies.

First is Castlereagh as "the embodiment of the sickness with which Ireland had infected British politics as a consequence of the union":[4]: 530–531  "We sent thee Castlereagh—as heaps of dead Have slain their slayers by the pest they spread".

"[4]: 531  For openly casting the same aspersions against the former Chief Secretary, in 1811 the London-based Irish publisher and former United Irishman, Peter Finnerty, was sentenced to eighteen months for libel.

[64] Some radicals, notably William Cobbett, claimed a "cover-up" within the government and viewed the verdict and Castlereagh's public funeral as a damning indictment of the elitism and privilege of the unreformed electoral system.