The abbreviations in the "category" column are:
A=anglicization. This means the name is an English re-spelling of an Irish Gaelic name (e.g., Kieran for Ciarán).
T= traditional name. This is an Irish Gaelic name that has been used in Ireland since before the 1800s.
Tr= Irish translation of a non-Irish name. These are Irish Gaelic forms of names whose origins are not Irish (e.g., Máire, for the Hebrew name Mary)
M= modern creation. These names were coined in the last century, or at least since the 1800s. Some of them are from Ireland, and others are from other countries with high Irish immigrant populations.
P= place name. These are the names of places in Ireland. They are generally not used for people, except outside of Ireland.
L= last name. These names are primarily surnames in Ireland, and are used as first names mostly outside of Ireland.
S= saint name. This is the name of an Irish saint.