Here are some titles that might be helpful here.

The Banshee: the irish supernatural death messenger by Patricia Lysaght (I found it).

The Old Gods: the facts about the Irish fairies by Patrick Logan.

The Truth about the leprechaun by Bob Curran.

Celtic mythology by Ward Rutherford.

Celtic myth and legend by Charles Squire.

The Fairy faith in Celtic countries by Wyevans Wentz.

These a few of the titles that I have. There are many, many more available.