Most workers for the Union Pacific Railroad were

Many of the Union Pacific railroad workers were young Civil War veterans, many were Irish immigrants, and almost all were single. The close attachment to the railroads meant a constant stream of transient residents and a mixing of ethnic groups under the banner of the Pacific Railroad.

Irish immigrants

Explanation: Many of the Union Pacific railroad workers were young Civil War veterans, many were Irish immigrants, and almost all were single.

C) Lasted two years.