Ancestry have added these detailed maps at an original scale of six inches to one mile.
According to Ancestry they
"offer an unparalleled look at 19th-century Ireland around the time of the Great Famine.
The English Parliament ordered Major Thomas Colby to Ireland in 1824 to undertake a survey of the country. His teams of surveyors would produce detailed maps on a six-inch = one-mile scale that would be used to determine land valuations for tax purposes. The maps were finally published in 1846. They cover almost the entire country and include details of the smallest civil division of the time: the townland.
This collection can be used in conjunction with several other databases on Ancestry.com, such as Ireland, Index to Griffith’s Valuation, 1848-1864. Researchers who find ancestors in other Irish databases can often locate where they lived on the ordnance survey maps.
Map sheets 108-125 are missing from the collection. These sheets cover approximately 10 percent of County Galway, mainly in the south of the county."The scale of the maps is such that you need a good grasp of the wider geography to use the maps effectively. It is a collection that warrants a more detailed description of the use in combination with other Irish resources than presently provided.