It is from an article in History and Theory (May 2005) titled Postmodernism and History: Missing the Missed Connections by David Roberts. The article is a peer review of a historiographymonograph. The sentence to which the topic text belongs is part of a description of a theoretical clash. The full sentence:
On one side, we have the postmodern challenge, which might seem to threaten the self-understanding of practicing historians, even to render history as a mode of inquiry irrelevant; on the other, the somewhat disparate response of those same historians to that challenge.
I second MuttQuad's opinion: 'render' means 'make' here, or as AHD also puts it, "to cause to become: The news rendered her speechless."
~ The topic text may be a little difficult to parse. A simplified version would be: The postmodern challenge (subject) seems to render irrelevant (predicate verb) history as a mode of inquiry. (direct object)