Periodical Source Index (PERSI) is an inventory of article titles from genealogical periodicals worldwide. It is not an index to the data within the article. Without this resource, valuable and essential research tools would be locked away in these journals. PERSI is the creation of the Allen County Public Library (ACPL) in Ft. Wayne, Indiana. They have the largest collection of genealogy periodicals in the United States. The Family History Library (FHL) probably has the second largest. The ACPL has one of the finest genealogy collections in the nation: www.genealogycenter.org/Home.aspx
I do not work on an Irish case without consulting PERSI. It is really that essential of a tool. Sometimes we forget that someone in Kansas may have extracted the very Irish record we need, and published their compilation in their local Kansas society journal. Without that title being inventoried in PERSI, by subject, locality or surname, it would be lost to us who do not receive that particular journal. The joy of PERSI is that it inventories such articles, no matter who publishes it. The prerequisite is that the ACPL has it in their collection.
The way I use PERSI is by geographic locality, such as Ireland and County Cork. I do use the name of the parish as a search parameter. Sometimes I will do a subject narrowed by county, such as tax lists.
Once you have the reference to the journal, article title, volume and number, then you can see if you have access to that issue close to you, at the FHL, or if you need to order a copy from the ACPL. There are many options for obtaining photocopies of articles. Some are even on the Internet.