Maybe I just pronounce it weird, but I tend to pronounce "iron" the same way as the phrase "I earn", just with less space between them. Obviously the "r" is closer to the "n" in my pronunciation, but it feels like a stretch to say that it's the "r" with the weird pronunciation. If anything, the vowels are what seem weird to me; the "i" is long, which does not seem obvious it would be the case from the spelling alone, and the "o" is just completely silent, which is the only case of that in English I can think of off the top of my head (although I wouldn't be surprised if there are a few others).
"iron" probably underwent a process called r-metathesis after the spelling was standardized (or the spelling was based on a different dialect). If the spelling had been based on the later version, it would've been "iorn". I was going to give a lengthy explanation, but someone on StackExchange already did: https://ell.stackexchange.com/a/264662