IRC is a legit organization, but that doesn't mean someone isn't trying to scam you. However, I think it's more likely that you just got mixed up with someone else.
Funny thing about donor-advised funds: when a charity receives a gift from a DAF, it often just shows up as a check from the bank that manages the fund with no indication of who made the gift. I don't believe the bank can disclose that information. Different orgs have different methods to attempt to keep track of potential donations so they are on the lookout, but it's not perfect. Somewhere out there is a generous donor who thinks that the rude IRC never thanked them for their gift.
As a side note, on behalf of all nonprofit fundraisers everywhere, I apologize for the unwanted snail mail. But we are going to keep sending it. Here's the problem: for many orgs, those annoying letters actually work, especially with the demographic that actually donates to charity in large numbers (65+).