From a genealogical point of view a Will can be of huge value in your research. As a bare minimum wills  provide the name, address and occupation of person to whom the will relates, the beneficiaries, witnesses to signing of the will and the date it was made.

Be aware that in nineteenth century Ireland the vast majority of the Catholic population were Tenant-at-will farmer or landless labourers with no real rights to the land they lived on and farmed. For this reason is unlikely for many to have had a will as they had little else in they way of valuable possession to pass on formally to next-of-kin.. Having said this, some did make wills, and with the Encumbered Estate Act of 1849 more gained ownership. This Act arose out of the famine, where tenants on an estate were unable to pay their rent due to the potato crop failure, and so in many cases the Landlord couldn't maintain his estate and had to sell it.

A Will must be authenticated, and this is done by a Court making a Grant of probate. If someone has not made a Will they are said to have died intestate.

Up until 1857 it was the responsibility of the Established Church; The Church of Ireland to deal with the administration of wills. It consisted of a network of Courts around the Country that carried out the work. Within each diocese there was a Consistorial Court which dealt with wills for the area. If the Will made reference to property of more than five pounds in another diocese then it was dealt with by the Prerogative Court in Armagh. This court was therefore  concerned with people of means, with large amounts of land in an area covering a number of diocese. So again, if researching a Catholic Irish Tenant farmer in the 19th Century, a will if any is more likely to have been dealt with by the Consistorial Court

After 1857 the handling of wills became of civil rather than church matter. 

The fire of 1922 meant the loss of much of the Wills records, but there are a number of Indexes to Irish Wills that provide good information.

The Ancestor Search Form will allow you to search indexes of  Wills from various parts of Ireland dating from the 1500's through the the mid-1800's.