It was implied that he had a girlfriend named Alice Jardine. In 1942, he was in the family's barn with her, and was about to make love with her, when his brothers came in. Sean yelled "Danny don't do it, you're a young man!" which startled Alice, causing her to run into the wall, knocking her out. Daniel got really mad, and tried hitting Sean with a shovel, but it hit a lantern, causing the barn to nearly catch on fire.
The next day, Daniel enlisted in the Army, and was originally assigned to a Company with his brothers in Able Company, 1st Battalion, 116th Infantry Regiment, 29th Infantry Division, but he was later transferred to a different Division. He was unseen during the movie but was reported to have been killed in New Guinea a week prior to D-day.
It is unknown where Daniel Ryan's final resting place is following the war, but it is likely he was buried in a war cemetery in New Guinea.