If you see the word theirs with an apostrophe before the ‘s’, it is incorrect. Theirs is a third person posessive pronoun used in place of their + noun, for example, if you say “the house is theirs” you could split theirs into their + noun which makes it “the house is their house“.
Putting an apostrophe before the ‘s’ is a common error. This could be because people link it to the way most other possessive pronouns are written like ‘Mandy’s coat’, ‘the book’s cover’ and ‘the dog’s mouth’. (These are all correctly written).
The Merriam-Webster online dictionary defines theirs as “that which belongs to them —used without a following noun as a pronoun equivalent in meaning to the adjective their “. The word has been around since the 14th century.
Note: another possessive pronoun where you wouldn’t put an apostrophe before the ‘s’ is in the word its (refer to Note 4 – The Apostrophe), e.g. ‘the bee didn’t go back in its hive’ and ‘the house and its surrounding area’. (These are all correctly written).
Other examples of possessive pronouns are the words yours (not your’s) and hers (not her’s), but I think I’ll leave those for another day.
So, to recap, the word theirs should never have an apostrophe before the ‘s’.
English plus (englishplus.com/grammar)
e Learn English Language (elearnenglishlanguage.com)
Note 331 – Cringing at the use of they’re, there and their… (mywritingnotebook.com)