One of my long term genealogical research projects has been to prepare a detailed narrative documenting the life of Colonel Augustus G. Tassin, U. S Army (1838 - 1893). Colonel Tassin is my second great-grand uncle. He was born in France, immigrated to America in 1852, and joined the 35th Indiana Volunteers in 1861, at the age of 23. He lived in Leopold Indiana at the time.
When I first got interested in Colonel Tassin, I happened upon a Yahoo Group devoted to the memory of the 35th (First Irish) Volunteers. I looked through the group's various pages and found some interesting history. I also found images of some typed daily diary pages made by an officer in the same unit; it wasn't the complete diary. They were detailed and made reference to Tassin. I copied them for later research use; I would someday search for the entire diary and probably use it in my report of Colonel Tassin.
Locating Hugh Gallagher's diary went on my "To Do" list and from time to time, I'd check listings of manuscripts and like documents for any clues. I never got serious about an all out search, knowing that I didn't need the diary right away. This is a long term project after all.
As I started writing about the various Civil War battles Colonel Tassin was involved in, came across a book written about ethnic regiments in the Union Army up on the Internet at Google Books. The book is Melting Pot Soldiers by William L. Burton. A quick scan of the bibliography uncovered several references like this one.
Over to the website for the the Indiana Historical Society, and this is what I find in the appendix of a Civil War Research Guide...
"...letters, regimental history, regimental records, correspondence, diary, visual item." Diary? Perfect, just what I've been looking for. This has to be Hugh Gallagher's Diary.
While the book that pointed me in the right direction can be viewed on Google Books, I bought my own copy - gently used - for less than what it would have cost me in fuel, parking, and time to view it at the local university. There are several other references to the 35th Indiana, with other sources that are worth follow-up.