I have three all brothers, all Irish from Boston. One served in the Navy on the USS Bermuda, one served in an Infantry regimnent but we cant figure out which one (possibly New York) and the final William O'Brien, who I know the most about and have letters by plus other personal effects.
In short william was working in New Hampshire when Sumpter was fired upon. Being 19 and youthful he enlisted two days later in company E, 1st New Hampshire Infantry and wore a gray uniform with red trim. Being one of Lincoln's first 75,000 90 day volunteers he did not see action. Once his enlistment expired he reenlisted back in Boston in Company I 24th Massachusetts Infantry "The NEw England Guards" as a Corporal.
He served in the Carolinas under Burnsides expedition early in the war and fought at Goldsburough and did island hopping of the South Carolina coast. He was in the same division as the famous 54th Massachusetts Colored Infantry in the movie Glory so he was in the same engagements including Fort Wagner. He served in Florida and in Virginia in Butler's Army of the James on the Bermuda Hundred Campaign. His regiment took over 100 casualties at the Second battle of Deep Bottom where he is specifically mentioned in the regiment history (now a 1st Sergeant) of his reaction is seeing his best friend shot in the head.
Once Richmond fell his regiment garrisoned Richmond up until January 1866 before being mustered out. The 24th was the second to last New England regiment to leave the army. During this time he rose to the rank of Captain and was the Union Commandant of the notorious formal Rebel Prison Libby Prison. It was considered the Andersonville for officers. It was essentially a warehouse on the Richmond wafs where Union officers were sent to rot. When it was turned over to Union control war criminals were sent there and watched over by William. He writes of having to persuade former inmates of not killing the Confederate commandant who was locked up there out of revenge. He is even mentioned int he Richmond Examiner as running "Captain O'Brien's Hotel"
Then in the early 1870's he went out to California and was blown up in a mining explosion. My family did not try to go to California again for another 160 years.