Mirror posts look like
https://mirror.xyz/0x58E3a8Bc8CBFC10AC2972Fb9d0cF359E21eae56b/ZIO--5ywx1z-aKs0KCQ2PeTNutVKune7zhA1D09R0q0
0x58E3a8Bc8CBFC10AC2972Fb9d0cF359E21eae56b
is the ETH address
ZIO--5ywx1z-aKs0KCQ2PeTNutVKune7zhA1D09R0q0
is the Original-Content-Digest, a unique id mirror uses to store posts on Arweave
The easiest way to retrieve the arweave transaction associated with a mirror post is to use arweave's graphql endpoint: https://arweave.net/graphql