Between 1775 and 1783 Robert Stewart lived in Bangor with his wife, while his father was living at Mount Stewart.

In 1776 a general election was held in Ireland.
Robert Stewart stood again for Down and was re-elected.
He sat until the dissolution of this parliament on 25 July 1783.

Stewart participated in the Irish Volunteers Movement which aimed to defend Ireland against foreign invasion when British troops were withdrawn to fight in the American Revolutionary War.
In 1779, worried by incursions of American and French vessels into the Irish sea, he organised the Newtownards Company of 115 men to act as fencibles.
In 1783 he attended the second Dungannon Convention.

His father, Alexander Stewart, died on 2 April 1781 and Robert inherited his father's properties in County Down.
He moved to the family seat, Mount Stewart, near Newtownards.
He completed the Temple of the Winds in the park of the residence, which his father had commissioned.
On 17 September 1782 he was sworn to the Irish Privy Council.

He stood in the 1783 general election for County Down but lost to Arthur Hill and Edward Ward.

In 1789 Robert Stewart was created Baron Londonderry in the Peerage of Ireland.

The following year, 1790, was another general election in Ireland, the occasion to wipe out the electoral defeat of 1783.
Being now a peer he could not stand for the House of Commons himself.
Instead, he took his eldest son, Robert (the future Viscount Castlereagh), out of Cambridge University and had him run for County Down, which he did successfully but at a cost of £60,000.

In 1795 Stewart became Viscount Castlereagh.
The following year, on 8 August 1796, he was elevated to Earl of Londonderry.

His second son, Charles, at this point also gained a seat in the Irish House of Commons as member of parliament for Thomastown borough, County Kilkenny, in an by-election to replace George Dunbar,
Lord Londonderry, as he was now, and his two sons favoured the Act of Union and fought to have it presented once more after it had been turned down in 1799.
With the passing of the Act of Union Londonderry become one of the 28 original Irish representative peers and therefore obtained a seat in the House of Lords in London, which he never took up.

1803–1806 he remodelled Mount Stewart house after a design by George Dance.

In 1816 already in his seventies he was created Marquess of Londonderry, due to the influence of his son, Lord Castlereagh.
This title is again in the Peerage of Ireland.
From 1801 until his death he was an Irish representative peer.
He achieved the rare feat of rising from a commoner to a marquess.
Between 1775 and 1783 Robert Stewart lived in Bangor with his wife, while his father was living at Mount Stewart. 
In 1776 a general election was held in Ireland. Robert Stewart stood again for Down and was re-elected. He sat until the dissolution of this parliament on 25 July 1783. 
Stewart participated in the Irish Volunteers Movement which aimed to defend Ireland against foreign invasion when British troops were withdrawn to fight in the American Revolutionary War. In 1779, worried by incursions of American and French vessels into the Irish sea, he organised the Newtownards Company of 115 men to act as fencibles. In 1783 he attended the second Dungannon Convention.
His father, Alexander Stewart, died on 2 April 1781 and Robert inherited his father's properties in County Down. He moved to the family seat, Mount Stewart, near Newtownards. He completed the Temple of the Winds in the park of the residence, which his father had commissioned. On 17 September 1782 he was sworn to the Irish Privy Council.
He stood in the 1783 general election for County Down but lost to Arthur Hill and Edward Ward.
In 1789 Robert Stewart was created Baron Londonderry in the Peerage of Ireland.
The following year, 1790, was another general election in Ireland, the occasion to wipe out the electoral defeat of 1783. Being now a peer he could not stand for the House of Commons himself. Instead, he took his eldest son, Robert (the future Viscount Castlereagh), out of Cambridge University and had him run for County Down, which he did successfully but at a cost of £60,000.
In 1795 Stewart became Viscount Castlereagh. The following year, on 8 August 1796, he was elevated to Earl of Londonderry. 
His second son, Charles, at this point also gained a seat in the Irish House of Commons as member of parliament for Thomastown borough, County Kilkenny, in an by-election to replace George Dunbar,
Lord Londonderry, as he was now, and his two sons favoured the Act of Union and fought to have it presented once more after it had been turned down in 1799. With the passing of the Act of Union Londonderry become one of the 28 original Irish representative peers and therefore obtained a seat in the House of Lords in London, which he never took up. 
1803–1806 he remodelled Mount Stewart house after a design by George Dance.
In 1816 already in his seventies he was created Marquess of Londonderry, due to the influence of his son, Lord Castlereagh. This title is again in the Peerage of Ireland. From 1801 until his death he was an Irish representative peer. He achieved the rare feat of rising from a commoner to a marquess.
Robert Stewart, 1st Marquess of Londonderry