Final answer:
Irish immigrants made up a large portion of the Union Pacific workforce during the construction of the Transcontinental Railroad.
Step-by-step explanation:
The immigrant group that made up a large part of the Union Pacific workforce was the Irish immigrants. During the construction of the Transcontinental Railroad, which included the Central Pacific Railroad section and the Union Pacific Railroad, different groups played various roles. The Central Pacific employed many Chinese immigrants, especially in challenging sections like the Sierra Nevada mountains. In contrast, the Union Pacific workforce was largely composed of Irish immigrants, among others such as African Americans and former soldiers.
These immigrant laborers faced incredibly grueling work conditions, often risking their lives to build the infrastructure that would unite the country. Despite the hardships and the racism, these workers contributed significantly to the economic growth and westward expansion of the United States.