The Public Record Office of Northern Ireland (PRONI) has made available an historical maps viewer. One can find sites, buildings and landmarks of historical interest in the six counties of present-day Northern Ireland: Antrim, Armagh, Down, Fermanagh, Londonderry, and Tyrone. The maps are from the six-inch Ordnance Survey County Series and the more recent Irish Grid. The PRONI site provides good instructions on how to use the various functions: www.nidirect.gov.uk/services/search-proni-historical-maps-viewer. Happy searching!