Finding out that you have an ecclesiastical figure in your family tree can be very useful for discovering your ancestor's place of origin in Ireland. In the Catholic Church, some form of study at a seminary is required to become a deacon, priest, or bishop. The first seminaries in the U.S. date back to the early 19th century and, if they survive, their records can be of value.
After ordination, members of the clergy were often profiled in parish publications or historical overviews of the Catholic Church in a particular city of state. For example, for more information about this topic in New York City, see Chapters 4 and 7 of my book, Finding Your Irish Ancestors in New York City.
That self-promoting segue brings me to an interesting title I recently came across, Bishops of the United States of Irish Birth or Descent. Chances are, if you have a Catholic bishops in your family tree you more than likely already know about it. If you don't know, or do, but don't know about their ancestry, then this book is worth a look. It contains a valuable amount of genealogical information about the 466 bishops profiled, from John Carroll, appointed in 1784 as Bishop of the Apostolic Prefecture of the United States (it became the Diocese of Baltimore in 1789), to John Ford, appointed as Bishop of Kaying, China, in 1935, the year before the book was published.
|Entries from p.38 of Bishops of the United States of Irish Birth or Descent|
When known, information is provided about the birth date and place of the bishop, where his parents were from, surname history information, and other useful nuggets about their origin. The chapter, Mothers of the Bishops, is particularly innovative, and provides the maiden names of all, bar three, of the bishops. There is also an Irish county and U.S. state breakdown of where bishops were born. Up to 1935, Tipperary and Massachusetts lead the way with twelve and twenty-nine, respectively.
The book is available to read on the excellent Hathi Trust website. If anyone has a bishop in their family tree it would be very interesting to hear from you in the comments section.
EDIT (25 April): A commenter on a previous post made the excellent point that information in these types of publications should be treated just the same as any secondary source. Citations are not provided for the claimed genealogical information. Use them as a guide in your research and be prepared to find evidence to the contrary of what is published in such volumes.
 Hackett, James Dominick. Bishops of the United States of Irish Birth or Descent. New York, NY: American Irish Historical Society. 1936.