Here is information shared by Brian Mitchell of the Derry Genealogy Centre:
"View, free of charge, the historic 19th-century mapping archive of Ordnance Survey Ireland.
Simply pan across the map, select map series (such as Historic 6", Historic 6" B&W or Historic 25") and zoom in to view the townland in greater detail. Historic 6" refers to 6-inch mapping series, both color and greyscale, of 1837-1842, and Historic 25" to 25 inch mapping series, greyscale, of 1888-1913."
(Thank You to Genealogical Publishing Company)