However, the comments that got the most attention from the teacher were written by several boys who walked down the road to the observation tower, which is located where the Irish Brigade charged the Confederate line. The boys described hearing strange noises that became shouts, coming from the field near the tower. Some of them said that it sounded like a chant and others described the voices as though someone were singing a Christmas song in a foreign
language -- a song like "Deck the Halls". They said the words sounded like a Christmas carol or something. It sounded like "Fa-la-la-la-la". The impression came strongly and then faded away.
Now the Irish Brigade was one of the most famous units of the entire Civil War. Both sides had Irish Brigades and both units had one thing in common. They were among the units with the heaviest casualties in the war. The reasons were because they lacked training. Many were taken right off of the boats from Ireland when they had a uniform and a gun slapped into their hands and were sent to places they had never heard of before. The Irish Brigade at Antietam was a Union unit. They rode with their priest, Father Corby. The man showed no fear as he would minister to the dead and dying. They also rode with their Emeral banner. During the battle, the flagstaff was actually shattered in a man's hands by gunfire. The Irish Brigade would arrive with the sounds of drums and a battle cry that sounded like "Fah-ah-bah-lah", which is Gaelic for "Clear the Way!" and is spelled Faugh-a-Balaugh. Of course, the cry would grow softer as they passed away. As a matter of fact, they lost 60% of their unit in Antietam in a few short, bloody hours.
So, its safe to assume that the singing the boys heard wasn't a Christmas song at all, but the sound of the Irish Brigade trying to clear the way.