IIRC is an internet acronym that has quite a few variations, all meaning the same thing.
Acronyms are abbreviations of words used to cut down on the time it takes to type. Acronyms are often typed and easier to understand when typed in capital letters.
There's no shame in seeing an acronym you don't recognize, and it's happened to everybody (or it eventually will).
IIRC means “If I Recall Correctly” or "If I Remember Correctly"
IIRC is a way to tell someone something from memory while leaving a little wiggle room that you might be wrong.
As mentioned, you can use similar acronyms to achieve the same result; however, for some reason, they never really caught on, so the person on the other end might not understand. Unlike most popular acronyms, if you search for what they mean, you won't find the results easily because some organizations already use the same letters and lack widespread acceptance.
They are:
IIAC - If I Am Correct IIANM - If I Am Not Mistaken
IIRC isn't that old, first appeared in the Urban Dictionary in 2002.
Urban Dictionary also lists one of the most disgusting acronyms we've ever seen for IIRC that seems out of place.
IIRC, like many acronyms, can be used as lowercase or uppercase.