Priority is based on two things - Move order (from 1 to 4) and Initiative (From refinement). That's all there is to it. If your iruka is in Move 4, you can bet he won't be interupting anybody =)

Ok, so interuption works on regular attack order, but do prompts also work on that order.

- so its 1-1-2-2-3-3-4-4 for attacks but do prompts follow that order. also, midnight blades skill in back row always start before my iruka in front row, its weird

prompts also follows that order

There are no odds when it comes to who goes first, you can figure it out by initiative and position.

Midnight Blades usually are not on the 1st position. So if you want to stop one, it's as simple as making sure someone who can interrupt is on position 1. Position 1 is indicate in the team screen, there's numbers next to each ninja telling the order they act in. Iruka needs to be 1 in order to act before EVERYONE who isn't also position 1.

Initiative is just a tiebreaker when two people at the same position number are trying to use a jutsu with the same casting speed.

Typically Midnight Blades are position 2 behind some sort of tank. If your interrupter is position 2 then he has to compete with the Initiative of a Main, which is usually very high. So the best way to interrupt him is to be in position 1. If you put Iruka in the top right corner next to your tank then he will be in position 1 and act faster than the enemy's Midnight.