To get it, Hannon and others waged a dogged campaign with the National Park Service, which has sought to prevent the battlefield from becoming overrun with memorials to various groups.
"We've had a hell of a time fighting the National Capital Park Service in getting it up," Hannon said.
Hannon said he first tried to get permission to erect a monument 11 years ago.
Park Superintendent John Howard said a general management plan in the early 1990s determined that the battlefield should be closed to future monuments. An exception was made for the Irish Brigade monument because negotiations already had begun, he said.
The last monument erected on the battlefield was a tribute to the state of Texas in the cornfield. It was set up in 1962, Howard said.
"We haven't had one since," he said.
Howard said the management plan seeks to strike a balance between memorializing the battlefield and preserving it.
"It's not that we take monuments lightly. We feel the best kind of monument we can have for the soldiers who died here is keeping the battlefield the way it looked when it was fought," he said.
The monument has a granite base with two bronze sculptures depicting a wounded soldier about to fall and another reaching over to carry the unit's green colors.
The Irish Brigade was made up mostly of Irishmen who fled Ireland's potato famine. At Antietam, the brigade attacked a well-defended position on Sunken Road, now known as Bloody Lane. During fierce fighting, 540 Irishmen were killed or wounded.
Hannon said he hopes the $150,000 monument will show Americans some of the contributions the Irish have made to the nation - accomplishments he said are often overlooked by historians.
"The Irish have done so much for this country," he said.