The title of Earl of Londonderry has been created three times in the Peerage of Ireland.

The first creation was for Thomas Ridgeway, who served as Treasurer of Ireland and was involved in the colonisation of Ulster. He was created a baronet in 1611, and Lord Ridgeway, Baron of Gallen_Ridgeway, in the Peerage of Ireland, in 1616. He was created Earl of Londonderry in 1622. All his honours became extinct in 1714, on the death of his great-grandson the fourth Earl.

The next creation was for Thomas Pitt, uncle of William Pitt the Elder. He married the daughter of the fourth Earl of Londonderry in 1717, and was created Baron Londonderry in 1719 and Earl of Londonderry and Viscount Gallen-Ridgeway in 1726, both in the Peerage of Ireland. The titles became extinct in 1765, on the death of his younger son the third Earl.

The third and present creation was for Robert Stewart. He was created Baron Londonderry in 1789, Viscount Castlereagh in 1795, Earl of Londonderry in 1796 and Marquess of Londonderry in 1816, all in the Peerage of Ireland. The titles are still extant.

Earls of Londonderry (1622)

• Thomas Ridgeway, 1st Earl of Londonderry (c. 1565_1631)
• Robert Ridgeway, 2nd Earl of Londonderry (d. 1641)
• Weston Ridgeway, 3rd Earl of Londonderry (1620_1672)
• Robert Ridgeway, 4th Earl of Londonderry (d. 1714)

Earls of Londonderry (1726)

• Thomas Pitt, 1st Earl of Londonderry (d. 1729)
• Thomas Pitt, 2nd Earl of Londonderry (d. 1734)
• Ridgeway Pitt, 3rd Earl of Londonderry (1722_1765)

Earls of Londonderry (1789)

• Robert Stewart, 1st Earl of Londonderry (1739-1821), created Marquess of Londonderry in 1816 (see that title for subsequent Earls)

Source: The Complete Peerage, Vol. VIII, article "Londonderry"