You have probably explored the Griffith’s Valuation pages here: http://askaboutireland.ie/griffith-valuation/.
This page explains the references on the maps: http://askaboutireland.ie/reading-room/history-heritage/irish-genealogy/understanding-the-valuati/
On Griffith’s Valuation, the numbers and letters of reference to the maps are the connection between the Valuation and the accompanying Ordnance Survey map; they appear on the left of the ‘Number and Letters’ column on the original pages of the Valuation. In general, each townland is surrounded by a thick line on the map, with the numbered subdivisions outlined with lighter lines.
The numbers represent a lot number, that is to say, a single area outlined on the Ordnance Survey map and labeled with that number.
Capital letters after the subdivision number (e.g. ‘2A, B ,C’) are used to label subdivisions within a lot and indicate separate parcels of property in the townland held by the same individual. For example, if a John Kelly rented two separate fields in the townland of Ballymore, these will be listed within the townland under his name following each other as 2 A, B. It follows that the order of the personal names within each townland does not imply geographical location: the fact that two names appear beside each other does not mean that the individuals were neighbours. The number represents only the order in which the valuator listed each holding in his manuscript field book.
The same capital letter can appear more than once within the same numbered lot when the location of cottagers’ or labourers’ houses is being indicated.
Lower-case letters after the holding number (e.g. ‘2a, b ,c’) indicate a single property held in common by a number of listed occupiers. This was common in rural areas in early and mid-nineteenth century Ireland , especially in the West, with anything up to 20 families farming an area in common.
Lower-case italic letters are used to indicate built structures, including houses. The order in which these lower-case letters appear is significant: where cottagers’ or labourers’ houses are included within the limits of a farm, the farmer’s house is labeled a, while the cottagers’ houses are labeled b, c …
It should be noted that the connection between the maps and the Valuation is never perfect; there are regular omissions and mistakes. The best connection is in the working copies of the maps used in the Valuation Office itself.