Irregular noun is is a noun that becomes plural by changing its spelling in other ways than adding "-s" and "-es" to the end of the word. the best example of irregular nouns are the that ends in "f".
Examples of irregular nouns:
elf - elves
leaf - leaves
loaf - loaves
wolf - wolves
shelf - shelves
person - people
man - men
child - children
goose - geese
tooth - teeth