Answer:
By definition, anything that is irregular does not follow rules. ('Regular' means 'conforming to a rule', so 'irregular' means 'not conforming to a rule'.) You can master the rules for forming the various types of regular plural, but the only way to learn how to form irregular plurals is to learn each one as you come across it.
Ox, oxen.
Sheep, sheep.
Child, children.
And so on ...