Tomie dePaola, this year's ALSC Wilder Award Winner, has an excellent book called Patrick: Patron Saint of Ireland which gives a biography of Patrick's life and work, as well as short versions of many of the famous myths and legends that surround him.
Here are some other St. Patrick's Day/Irish themed books that you might enjoy:
The Night Before St. Patrick's Day by Natasha Wing
St. Patrick's Day Alphabet by Beverly Vidrine
Fin M'Coul: The Giant of Knockmany Hill by Tomie dePaolaFinn MacCoul and His Fearless Wife illustrated by Robert Byrd
Irish Legends for the Very Young by Niamh Sharkey