'Folk' and 'Nationalism' are both abstractions. Give me something a little more specific, and I can answer your question. I do know that among British folkies, chauvinistic nationalism is anathema, notwithstanding the efforts of some on the right to latch onto folk music and folk dance.

Of course, there is positive 'nationalism' that is also associated with folk and folk-like things - being proud of your heritage does not necessarily mean disparaging or stomping upon someone else's.