Participating in their first real campaign, the 30th Missouri would prove its mettle as it was thrown against entrenched Confederate Infantry, only to be repulsed again and again during the battle of Chickasaw Bluff. Although the original campaign proved futile, a new campaign emerged during the army's withdrawal, and by January 1863, the Irishmen of the "Ole" 30th once again found themselves facing Rebel cannon during the Battle and siege of Arkansas Post, Arkansas. The post fell and the regiment did duty in northern Louisiana until May 1863, when they were ordered to join Ulysses S. Grants Vicksburg Campaign.
After engaging Rebel troops on Haines and Drumgoulds Bluff in order to mask the army's crossing of the Mississippi, the regiment moved to join the army in the rear of Vicksburg. Arriving at that great Rebel stronghold, the regiment paid a high price in blood during the two great assaults upon the Vicksburg defenses. But the boys of Missouri dug in and diligently manned the trenches until the eventual downfall of that place.
After the capture of Jackson, Mississippi, the 30th Missouri was sent to Vidalia, Louisiana, just opposite the river to Natches. While here they participated in several expeditions and repulsed two Confederate attacks, inflicting severe losses. Returning to Vicksburg in April of 1864, the regiment constituted a part of that city's defense until being sent to Arkansas to join the new 19th Corps.
In August, the 30th Missouri was joined by their fellow Irishmen of the 7th Missouri Veteran Volunteers. They were formed into a Demi-Brigade known as the "Missouri Irish Brigade". While in this brigade, the 30th Missouri performed mostly guard and occupation duty throughout the Confederate Trans-Mississippi.
After engaging Confederates around Clinton, Louisiana, the brigade was reorganized and absorbed into a larger brigade. The 30th Missouri was reduced to a Battalion of 4 Companies and not long after was Consolidated into a single Battalion of Missouri Infantry with the 3 Companies of the 7th Missouri. Although officially known as the "Consolidated Battalion 7th and 30th Missouri Volunteers", they were quickly labeled as the "Missouri Irish Battalion" by their peers and fellow soldiers. Their duty's varied little, as they continued guard and occupation duty throughout the Confederate Trans-Mississippi.
In December of 1864, the orders of Consolidation were reversed by the Federal High Command. With the transfer of the 7th Missouri, the 30th Missouri was ordered to join the army that was preparing to take Mobile, Alabama. On April 9th, 1865 the 30th Missouri participated in the assault and capture of Fort Blakely.
With the war all but over, the regiment did occupation duty throughout Alabama until mid-June, when ordered to Texas. They did duty at that place, until being mustered out in August 1865.