How did economic opportunities encourage the Chinese and the Irish to immigrate to the West Coast in the mid-1800s/early 1900s?

The Transcontinental Railroad was being built and the Irish immigrants were the first to work on the Central Pacific Railroad. However the pay was not high so many of the Irish immigrants left their jobs. To fill the much needed manpower, Chinese immigrants were hired. Most of them had come to California for the Gold Rush and did not go back home. By 1868, they represented 80% of the total labor force.