How to add a possessive to a plural that doesn't follow the "add s" rule?

I am upgrading a comment to an answer as requested by StephenS

It is generally true that forming the possessive of a plural that does not end in 's' involves adding an "apostrophe s". Examples are "The children's shoes", "The foremen's office" and "The sheep's faces". People get confused because we are taught, misleadingly, that the "s apostrophe" is used for plurals when it's only used for plurals that end in 's'.

There can be a small problem with words like 'sheep' and 'deer' where the singular and plural are the same, in this case it is impossible to tell whether the possessive is singular or plural. In most cases the context will make it clear as in "the sheep's faces" since most sheep only have one face. If you have a problem with this as in "the deer's legs" you can always replace it with "the legs of the deer" if you prefer, although even that will need context to show exactly what you mean.

As has been said in responses to other posts (such as this one from 2010) there are other complexities relating to singular nouns ending in 's' and particularly to Proper Nouns ending in 's' People get thmselves horribly confused by phrases such as "the boss's" car (correct, we are only talking about one boss so we add 'apostrophe s'), "the bosses' cars" (it's a plural ending in 's'), "Mr James's house" (it's a singular ending in 's') and "the Jameses' house" (the house occupied by Mr James, Mrs James, all the James children and, possibly, Mr James's elderly father) where the plural is 'Jameses' which sounds wrong. Again you can bypass the problem by saying "The James family's house".

There is a really confusing name in this context which is the royal court of the Sovreign of the United Kingdom, This is called The Court of St James's which is has the possessive 'of' but still has the possessive 'apostrophe s' at the end of James. The reason for this is that it is named after St James's Palace so it's not St James's court, it's the court associated with St James's Palace. Even more confusing, for people who have beeen told that the posessive 's' at the end of Proper Nouns ending in 's' is not pronounced, the possessive 's' at the end of James's is pronounced in this case.