what  are irregular   nouns 

While defining nouns we say the word which names a person, place, idea, thing or animal is a noun. So, when we think of making these nouns into plural form, we usually add an 's' in the word. However, there are many nouns which do not add 's' to become plural. Such nouns which have different rules to become plural are known as irregular nouns. Instances of the same are child-children, mouse-mice, sheep-sheep, etc.

Hope this explanation helped you understand the concept of irregular nouns clearly.