I was asked before which conjugation of 이르다 is correct, 이르러요 or 일러요. The person who asked had seen both of them and wondered whether one conjugation was preferable to another. The answer is that neither is preferable to the other because they’re different words!
이르다 has more than one meaning, and the two conjugations, 이르러요 and 일러요, come from two different meanings that are two different types of irregulars. Let’s take a look.