IIRC is popular on online platforms, such as social media, messaging applications, online forums, and emails. It comes up in discussions when one is struggling to remember information like dates, facts or discussion that has been made earlier. Such acronyms are often used on such platforms as chat groups, comment sections, and discussion boards. It is used casually  and is therefore not appropriate to a professional or academic response in which one would prefer to use more formal words. Know more about iirc meaning