While virtually any good map with sufficient detail can be useful, start off by locating maps of Co. Longford that show the parishes, the baronies, and the poor law union boundaries. Then see if you can find for the parishes of interest a map showing the townland boundaries. A great source of a map showing townland boundaries is the index map in David Leahy's Book Survivors of the Great Famine in Co. Longford. Whether or not you find all the above, you do need the modern 1:50,000 Ordnance Survey Maps for the area of Co. Longford of interest. Lastly, one should track down some form of the Victorian six inch to the mile maps. These can be downloaded from the subscription site Irish Origins. These particular maps are marvellous in that they have notations on land parcels that relate to those in the 1854 Griffith's Valuation. This may be a date after migration, but the land parcels still help sort out where family members lived. In late Spring 2011 Ancestry.com has added copies of the 6in. maps, but for better or worse they are the earliest versions, and without the land notations associated with the Griffiths Valuation. Note that ancestry.com also has the tithe applotments and Griffiths Valuations searchable. Unfortunately the tithe applotment listings are imperfect - So Rymsza's book is preferred.