"Τα μέρη" could also mean "the places". It is the plural form of "το μέρος".
Is μέρη a irregular plural form? I have never seen other words with plural ending with η. I am just a beginner, I haven't seen much Greek either.
Τhey are not irregular, the neuter gender nouns that end in -ος have plural ending -η. e.g. το δάσος (the forest) -> τα δάση, το μέρος -> τα μέρη.