It's not the verb "ирэх" itself that requires a subject in accusative case, but the verb's ending. For example: Чамайг явахад би уйлна. (I will cry when you go) Намайг очиход чи баярлах уу? (Will you be happy when I come?) Эгчийг ирэхэд хоол бэлэн байв. (Dinner was ready when sister came home) Болдыг харихад гэрт нь хэн ч байхгүй байв. (No one was home when Bold came back) So please check the meaning of "~ад /~эд /~од".