At the Battle of Antietam, the bloodiest day of the Civil war In eight hours 23,000 men would lay dead or wounded. Captain Edwin Field wrote:
"It was their crowning glory (The Irishmen had forced the enemy back beyond a sunken road) which had been filled with corpses by an enfilading fire from one of our batteries and presented one of the most ghastly spectacles of war.
Using this lane as a breastwork, they held it to the close of the fight, losing not a prisoner, having not one straggler, but at a loss of life that was appalling.
One Irish regiment lost nearly 50% of it men, another over thirty per cent. the rebels seemed to have a special spite against the green flag, and five color bearers were shot down successively in a short time.
As the last man fell even these Irishmen hesitated a moment to assume a task synonymous with death. "Big Gleason" Captain of the sixty third, six feet seven, sprang forward and snatched it up. In a few minutes a bullet struck the staff, shattering it to pieces; Gleason tore the flag from the broken staff, wrapped it around his body, putting his sword belt over it, and went through the rest of the fight untouched.
At the brow of the hill the fighting was the severest and most deadly ever witnessed before, so acknowledged by veterans in the service. Men on both sides fell in large numbers and those who were eye witness to the struggle did not think it possible for a single man to escape. The enemy here first, were concealed behind a knoll, so that only their heads were exposed. The brigade advanced up the hill with a cheer, when a most deadly fire was poured in by a second line of the enemy concealed in the Sharpsburg road, which at this place is several feet lower then the surrounding surface, forming a complete rifle pit and also from a force partially concealed still further to the rear.
The line of the brigade, in its advance up the hill, was broken in the center temporarily by an obstruction the right wing having advanced to keep up with the colors, and fell back a short distance, when General Meagher directed that a rail fence - which the enemy only a few minutes before had been fighting behind- be torn down.
His men, in the face of a galling fire, obeyed the order, when the whole brigade advanced to the brow of the hill, cheering as they went, and causing the enemy to fall back to their second line- which is some three feet lower then the surrounding surface. In this road were massed a large force of infantry and here was the most hotly contested point of the day. Each brigade of this division was brought into action at this point, and the struggle was truly terrific for more then four hours-the enemy finally however, were forced from their position. The Brigade suffered terribly. General Meagher horse was shot out from under him, and a bullet passed through his clothes. The sixty third regiment of this brigade, always conspicuous for deeds of daring in battle, was particularly so in the battle of Antietam.
The colors were shot down sixteen times and on each occasion a man was ready to spring forward and place the colors in front. John Hartigan, a member of company H, and only 16 years old, went some distance in advance of the regiment with the colors and waved them defiantly in the face of the enemy.
The whole Brigade gave a cheer that was heard along the lines for a mile, when it advanced up the rising ground and drove the enemy from a strong position"