Under the Irish Poor Law Act of 1838 commissioners were empowered to "unite so many townlands as they think fit to be a union for the relief of the destitute poor". These Unions were further subdivided into electoral divisions which were chargeable for the relief of their poor. In some instances individual estates were recognised as electoral divisions. Under the terms of the Act, a valuation of each separate tenement was authorised to meet the requirement of the assessment of the poor rate. This valuation was entrusted to the guardians of each poor-law union and was carried out by local valuators appointed by them. In many instances the local valuation was assigned to incompetent and dishonest valuators. The passing of the first Tenement Valuation Act and subsequent amending legislation placed the valuation on amore uniform basis throughout the country. Because of the role of Richard Griffith, mining and canal engineer and commissioner in charge of the Board of Works Relief Department during the Famine, in directing the Valuation it is commonly referred to as 'Griffith's Valuation'. Land and buildings were valued separately. In rural areas the valuation was carried out on a townland basis. In towns individual tenements were arranged according to streets.

These books were prepared for baronies which were subdivided into townlands, civil parishes and electoral divisions of the poor law unions. The numeral in the first column referred to the location of the individual tenement on the six inch to the mile townland maps. These maps, which show the boundaries of holdings on a townland basis can be misleading, inso far as subsequent changes were often superimposed on the original valuation maps. The division of a tenement into sub-tenancies was indicated by the use of letters and holdings-in-common were bracketed together. This enables us to estimate the extent of these practices on a barony basis. The names of the occupiers and lessors are of great importance. They allow the historical geographer to reconstuct patterns of land ownership and occupation and can also be utilised as a measure of either continuity or change.

The Valuation Books are a more comprehensive and thorough inventory than for example the great ledger books of the seventeenth century. These paid no attention to occupiers. In the 'description of tenement' a distinction was drawn between tenements which consisted of land only, and those on which a house or other buildings were located. From an examination of this information it was possible to determine the distribution and acreage of nonresidential holdings in Fassadinin. Buildings which served religious, commercial and administrative functions were also identified. However, the commercial functions of buildings were not always listed. Retail outlets for basic commodities such as tea and alcoholic drink, for example, were not identified. Information on the size, distribution and number of holdings permitted an evaluation of landholding patterns in the barony. By utilising these particulars it was also possible to indicate the areas in which, 'commercial' as distinct from 'subsistence' agriculture was practised. The net annual value of a tenement was defined as, "the rent for which one year with another, the same might in its actual state be reasonably expected to let from year to year with cost of repairs, insurance, maintenance, rates, taxes and all other. public charges except the tithe rent being paid by the tenement".

Land was valued at the current prices of the day, mainly with reference to the 'intrinsic' fertility of the soil and not according to the state of agriculture prevailing in 1850. 'Pasturage', was to be valued at the price per acre proportionate to the number of cattle and sheep it may be capable of grazing during the year, according to the usual price per head in the neighbourhood for grazing. The quality of the 'herbage' and 'permanent' improvements such as roads, drainage and fences were to be taken into account. The Valuators operated a sliding scale of allowances in respect of what were termed 'peculiar local circumstances'. When these circumstances, for example, relative location, communications, climate, elevation and shelter were favourable to agriculture, land values were correspondingly increased.

On the other hand when local circumstances were unfavourable, land values were reduced. Land valuation was, to an extent, an indication of soil fertility. It did however, take many other aspects of site and situation into account. It may be more correct to suggest that the valuation was an assessment of land potential as perceived by the valuators. The valuation of buildings was ascertained separately from land. Censuses compiled from 1851 onwards, contain a summary of the total valuation of townlands and civil parishes.