Castlereagh

• Marquess of Londonderry, subsidiary title Viscount Castlereagh, used as courtesy title for the Marquess's eldest son; in particular
  • Robert Stewart, Viscount Castlereagh (1769–1822), later 2nd Marquess of Londonderry: Anglo-Irish statesman and British Foreign Secretary.

In Northern Ireland:

• Castlereagh (borough), a local government district near Belfast
• Castlereagh (County Down townland) which gives its name to the borough
• Castlereagh (County Down barony), latterly divided into
  • Castlereagh Upper and
  • Castlereagh Lower

In the Republic of Ireland:

• County Mayo: townlands in two baronies:
  • Castlereagh, Clanmorris, Crossboyne parish
  • Castlereagh, Tirawley, Killala parish
• Castlereagh, County Offaly, townland in Lemanaghan parish, Garrycastle barony
• County Roscommon:
  • Castlerea, town
  • Castlereagh (County Roscommon barony), around the town
  • Castlereagh (County Roscommon townland), townland around the town
• Castlereagh, County Waterford, townland in Kilronan parish, Glenahiry barony

In Australia:

• Castlereagh, New South Wales, a suburb of Sydney
• Castlereagh River, two rivers
• Castlereagh Highway, a New South Wales state highway
• Castlereagh Street, a street in CBD Sydney

Other places:

• Castlereagh, Nova Scotia, a former village on the Bass River in Nova Scotia, Canada
• Castlereagh Dam and Reservoir, in Nuwara Eliya district, Sri Lanka

• The Castlereagh (Gilgandra), an early-20th-century newspaper in New South Wales, Australia