Derry, officially Londonderry, is the largest city in County Londonderry, the second-largest in Northern Ireland and the fifth-largest on the island …Derry, officially Londonderry, is the largest city in County Londonderry, the second-largest in Northern Ireland and the fifth-largest on the island of Ireland. The old walled city lies on the west bank of the River Foyle, which is spanned by two road bridges and one footbridge. The city now covers both banks.