The Know Nothing Party discriminated against Irish immigrants because their party was mostly comprised of

The Know Nothing Party, or American Party, organized as the political expression of nativism, hostility directed against German and Irish Roman Catholics, who immigrated heavily in the 1840s and 1850s. Nativism first impacted politics in the form of election-day riots provoked by secret fraternal organizations such as the Order of the Star Spangled Banner, organized in New York in 1849.

Answer: anti-Catholic Protestants