For the purposes of government administration in the 1830s, the Poor Law Unions (PLU) were created. There were a total of 163 PLUs and each had a workhouse associated with it.
Workhouses were the last resort for the destitute Irish. You may come across your ancestors with an address of the union workhouse. If this is the case, you can accept the fact that your ancestors had nothing so came to the workhouse to live, work and survive. They were horrendous places were families were separated based on gender.
With the introduction of civil registration for births, marriages and deaths in 1864, these PLUs were used as the basis for the Registration Districts (RD). When you see the Registration Districts listed on civil records and remember that this district is the same as the Poor Law Union. You can see this in the example below. On www.irishgenealogy.ie website, these districts will be displayed following your search on the left hand panel.
Getting used to these terms is a must to help understand the results being returned and are used in many other historical records.