Irimi as I have learned it is enter. Pure and simple. Enter. No specific path is implied other than that which allows me to control the center which varies a bit depending on situation. In eight basics as I was taught it sometimes it means directly entering ON centerline as for shomenuchi kokyunage, sometimes it means entering alongside as on tsuki kotegaishe. Where or from whom comes your belief it means off the line?
From the same place as you. Teacher. Then plus common sense. In order to blend with, thus Aikido.
Defined as passing behind.
Now if you also want to use it as passing straight through then so be it but in Aikido that would lead to confusions in communication and people doing irimi straight into a spear.
I don't believe you or anyone does move straight into anyway unless you're hitting someone with your chest or belly. Or maybe a head butt.