Often people miss the very person they are looking for in Irish records, because they are not aware of variations in given names. For example, if you didn’t know Delia, Bedelia or Cordelia are nicknames for Bridget, then you may skip over the marriage or christening record you need. Another one you may not have thought about is that Eugene and Owen are interchangeable. If you didn’t know these nicknames, then it could cause you all kinds of grief in your research.
My rule of thumb is that most male names are somewhat straightforward. Corny is a nick name for Cornelius, as is Con and Neil. Those make sense. Perhaps a less straightforward one is Darby or Dermot for Jeremiah. However, the female names can be complicated from two perspectives.
The first perspective is that you might find Mary as Catherine or Bridget, with no rhyme or reason why. Those can be frustrating. From the second perspective, Johanna, Hannah, Anna, Hanora, Honora and Nora are the same name. In this second example, you would probably figure it out along the way during the research process.
There are many books on the market for Irish given names. The one I use all the time was written by genealogist Rosemary Muscarella Ardolina, What’s in a Name? A List of Christian Names, and Their Irish Nicknames, Variants, Irish & Latin Equivalents (Floral Park, New York: Delia Publications, 2001). I would highly recommend you purchase this small 22 page book: http://deliapublications.com/index.htm