The 116th PA fought as part of the Irish Brigade: the 2nd Brigade in the 1st Division of II Corps, which fought on the Union right at Fredericksburg. The Irish Brigade had a hard-won reputation as some of the strongest fighters in the US Army, and they paid for that reputation in blood; only two other brigades in the army lost more men over the course of the war.

The 116th PA had a strong Irish element, and was led by an Irish colonel, but it also had a large number of Germans in the ranks, and Fredericksburg would be their first time fighting under the green banner of the Irish Brigade. At this point in the war, the Union had not implemented conscription; these were volunteers, and really, most of both armies were made up of men who had volunteered in the first year of the war, though the 116th PA was formed in the second year, after the recruitment offices were reopened after having been closed in the spring.

The night before the battle, the 116th bivouacked with the rest of the army on the ground around Fredericksburg, on the southern bank of the river. Virginia isn't exactly an arctic wasteland, but sleeping without shelter in early December is never a pleasant experience. "Private William McCarter of the 116th Pennsylvania recalled that night as “sleepless and most uncomfortable,” with the only rest coming after he separated the staves of an old barrel, laid them out, and found some small protection from the damp ground. St. Clair Mul- holland fared no better, declaring it “one of the most dismal and miser- able [nights] ever experienced.” He noted, “The troops [were] massed so close that there was not even room enough for the men to lie down on the ground, and it was a fortunate man who could secure a cracker box to sit upon during the weary hours. Sleep was impossible, it was so cold and chilly.” McCarter appears to have been a lucky one, managing an hour’s sleep on his broken barrel."

Before the attack of the brigade began, their general, Meagher, addressed the units in turn, reminding them of their duty to their adopted homeland, though during his pep talk, several men had been killed when a Confederate shell landed in their midst. They stepped off the advance after having entwined sprigs of boxwood in their caps; the brigade's green banner had been shredded in previous engagements, and was in New York while a replacement was being brought in. The men needed to show off something green to let the world know these were Irishmen.

Before even seeing the enemy, the Brigade came across the wounded from previous attacks; one Irish veteran was considerate enough to step out of the ranks to saw off the mangled leg of an officer, which was then hanging by a single tendon. Pvt. McCarter noted African American people scrambling through the streets of Fredericksburg as artillery crashed into the town, making a special note of a woman with three children; a shell struck them, killing her and two children, while leaving the third badly wounded. "St. Clair Mul- holland declared the trail of wounded along Sophia Street a scene “not calculated to enthuse the men or cheer them.” "

At Fredericksburg, the 116th's position on the Union right put them in position to attack the Confederate left, which was based on Mayre's Heights southwest of Fredericksburg. This gave the Confederate defenders the high ground, and allowed them to take cover behind a stone wall; it was going to be a tough nut to crack. After they crossed Princess Anne street, there was no shelter on the plateau from the hurricane of fire the Confederate were laying on the fields. Mulholland recalled when the 116th PA was marching past the free black neighborhood Liberty Town, one of his sergeants, John C. Marley, was instantly decapitated by a cannonball, and fell still clutching his musket. He furthermore witnessed three men struck down by a single ball.

A small waterway ran behind Fredericksburg, called the millrace; while the 69th and 88th New York and 28th Massachusetts crossed using the remains of a bridge over it, the 116th had to wade through the frigid water (it was about 30 degrees that morning, though it had warmed up to about 60 during the day) to get across faster. It took the brigade about half an hour to cross the obstacle, after which they formed into line of battle and fixed bayonets. After seeing them across, their general retired to the rear to get his horse, having had pain in his knee.

As the Irish pressed forward, they saw the previous brigade breaking up ahead of them, and entered into a furious hail of shot and shell. While there were Irish units in the Confederate lines opposite them, this didn't stop them from carrying out murderous execution by musketry. Pvt. McCarter of the 116th PA was wounded twice in a few minutes; a bullet struck his shoulder, and after another left his cap dangling by a thread from his ear, a second bullet struck his arm, and he fell unconscious. St. Clair Mulholland saw his fellow officers cut down one after another; a bullet tore through one lieutenant's thigh, while another punched through another lieutenant's lungs. He recalled seeing a sergeant spraying blood from a hole in his forehead, and was himself struck down by a bullet in the calf. While the 69th NY was able to get within 50 yards of the Confederate position, the attack had failed, and leaving the wounded where they lay, the survivors hugged the ground as they retreated from Confederate fire. The brigade's historian remarked it had barely risen to the level of battle, and that slaughter was perhaps a more accurate term.

The 116th PA took especially heavy losses; the brigade as whole had lost about 40% of its officers and men during the attack, but the 116th had left more men on the field than were still in the ranks. As a whole, the brigade could only muster 250 men after the battle. To put that in perspective, 300 men was the average strength of a single regiment by Gettysburg, with a paper strength of 800.

Unsurprisingly, the battle was a dispiriting experience for the men of the unit. This was the first major battle fought after George McClellan, who had built the Army of the Potomac and inspired its men, was removed from command, and he was sorely missed; the newspaper Irish-American published a letter from a captain in the 88th NY, which praised his scientific approach to battle, the devotion he inspired in the men, and the humane care he showed for their lives. "We are slaughtered like sheep, and no result but defeat."

I don't know your family background, so I don't know if this is relevant, but many of the Irish in the Union army were actually fighting for Ireland; they saw service in the US Army as a way to gain military experience for a future war to liberate Ireland from the British.

If you'll permit some dark humor, I hope your ancestor patronized the embalmers who accosted the Irish Brigade before the battle, "handing them business cards with promises that should any man require “an early trip home” he would be “nicely boxed up and delivered to loving friends by express, sweet as a nut and in perfect preservation.” "

All quotes taken from Susannah Ural's incredible book, The Harp and the Eagle: Irish American Volunteers and the Union Army, 1861-1865.