OSNI
Ordnance Survey of Northern Ireland covers Northern Ireland
View OSNI mapping on-line
OSI
Ordnance Survey of Ireland covers the Irish Republic
View OSI mapping on-line
Short Histories by Dr. Richard Oliver
Other Related
See OSI website for more information about Ordnance Survey Ireland
See OSNI website for more information about Ordnance Survey of Northern Ireland (Now part of Land and Property Services)