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.
Londonderry is included in the north-west circuit; the assizes are held in the city, and quarter sessions are held there and at Coleraine, Newtown Limavady, and Magherafelt.
Besides the remains of monastic institutions in the city of Londonderry, seventeen others appear to have existed within the limits of the county; there are still remains of those situated respectively at Camus, Errigal, Tamlaghtfinlagan, Domnach Dola, and Dungiven, at the last of which are the most interesting of all the ecclesiastical ruins.
The lunatic asylum for the counties of Londonderry, Donegal and Tyrone, situated on rising groung to the north of the city was commenced in June 1827, and opened in 1829; the entire expense, including the purchase of the site and furniture amounted to £25,678.