The Home Menu
downloads a list of all updated apps over SpotPass (versionList.dat) and stores it on the system NAND, so blocking the update server (e.g. with the Tubehax DNS) isn't enough to stop these app update nags, which are served from a different location. Additionally, once you start getting these nags, no amount of server blocking will stop them, because the 3DS is checking the file stored on NAND, not the server.
If you haven't received the nag for Ironfall yet, you likely don't have the most recent versionList.dat. If your router allows blocking domains, you could try blocking
https://tagaya-ctr.cdn.nintendo.net/. But again, this won't stop Ironfall nags if you're already getting them.