The following collections of sources provide a great starting point when you're trying to find out more about a topic so that you can dig deeper (think Wikipedia but professor-approved!).
Some tips: When looking at sources aimed at general overviews, isolate your search to as few keywords or phrases as you can. For example, search just for "whiteness" rather than "irish-americans and whiteness." Start broadly, and then use some of the sources listed, or the subtopics mentioned, to find a direction that best suits your project and personal interests.