In this article, we shall endeavor to answer the following three queries: First of all, what are the relations between Marxism and Nationalism, or those between class and nation/ethnic group? Further, how had Karl Marx understood British colonialism in Ireland? In other words, how had he diagnosed Ireland's fate and what had he prescribed for the Irish? Finally, we shall look into how Marx's disciples had treated nationalism. In conclusions, we shall compare the later development in Ireland with what Marx had predicted.