Theory:

Plural noun rules for Irregular nouns
Irregular nouns follow no specific rules
Example:
 Singular
 Plural
childchildren
goosegeese
manmen
womanwomen
toothteeth
footfeet
mousemice
personpeople
Two plural forms with different meanings:

1. Both forms follow the English pattern:
Example:
i. cloth - cloths (i.e. 'some pieces of material to wipe something up'); clothes (i.e. 'garments')
  
2. Foreign plurals occur along with regular plurals:
Example:
i. genius - genii (i.e. 'supernatural beings') - geniuses (i.e. 'very intelligent persons')
 
Irregular Verb/Noun Agreement:
  
1. Some nouns have a plural form but take a singular verb.
Example:
i. news
ii. billiards
2. Some nouns have a fixed plural form and take a plural verb (Plural noun with a plural verb).
Example:
i. jeans
ii. glasses
iii. trousers
  
Possessive plural nouns:
The possessive of a plural noun is formed by adding only an apostrophe when the noun ends in 's', and by adding both an apostrophe and 's' when it ends in a letter other than 's'.
Example:
i. Excessive lawyers’ fees
ii. Children’s toys
iii. The twins’ parents