William Monaghan in The Cornfield is not a fictional character. Since I know so little about him, I took several literary licenses with him. He was, though, Irish born, a notary in New Orleans, and in his 40s when in the 6th Louisiana Volunteers. He did recruit Irish soldiers into the regiment at the Olive Branch Coffee House. He got married on the eve of his enrollment in the army at Camp Moore.
Monaghan started the war as a captain and commander of Company K. He was promoted to major while imprisoned at the Old Brick Capitol. After recovering from his wounds at Chantilly, he commanded the 6th Louisiana regiment. After Antietam, he was promoted to colonel and commanded the entire Louisiana brigade.
Monaghan was killed on August 29, 1864, while leading a charge near Shepherdstown, in the newly formed State of West Virginia, near the Potomac River. He is buried there.
The photo at the top of the page is from a wartime New Orleans newspaper.