The phrase "irreplaceable part" means something or someone that is unique, valuable, and cannot easily be replaced or substituted with something else. Full definition
It so quickly received vast popularity assisting people to end with their lonely life that became almost irreplaceable part of their life. (date4plus.com)
Sixty years ago, Catholics played a prominent, prestigious, and irreplaceable part in American literary culture. (firstthings.com)
«Diane was not simply present to oversee every one of the stages of this book's revision; she was a crucial and irreplaceable part of its conception. (thebookguru.org)