On July 31, 1838, the Poor Law act was enacted which divided Ireland into Poor Law Unions. By 1847, there were 130 unions and later some of these were divided into two or more unions so that by 1864 there were a total of 163 unions.

In each PLU there was established a workhouse, with a market town as its center, the union usually had about a ten mile radius boundary. These boundaries had no relation to baronie, county or civil parish boundaries. The purpose of the union was to collect rates from the inhabitants from the inhabitants to provide for the poor and destitute living in that union.

In 1864, when the civil registration of Births and Deaths in Ireland was made compulsory for the country, the Poor Law Union was used as a Superintendent Registrar's District (SRD), and within each district, a Dispensary/Registrar's (RD) district was created. If searching for a birth, marriage or death certificate, it is important to know the SRD when ordering the certificate.

In 1885 a series of pamphlets were printed that listed the townlands located in each Poor Law Union. They are arranged in SRDs. In the SDR they are further divided by RD and then by Electoral Division (ED) which is useful to know when searching the censuses and land records. The Barony and parish are further designations. The below information is taken from copies of the pamphlets reprinted in a book called, Townlands in Poor Law Unions by George B Handran in 1997.

Athy PLU:   161,130 area in miles; Population in 1881:  27,961
(all townlands in this PLU are located either in the county of Kildare or the county of Leix)
Abbreviations:  D=Registrar's District, E.D.=Electoral Division, B.=Barony, P=Civil Parish

Abbeyleix PLU: 127,167 area in miles; Population in 1881: 18,181
(all townlands in this PLU are located in the county of Leix)
Abbreviations:  D=Registrar's District, E.D.=Electoral Division, B.=Barony, P=Civil Parish