When a name is common across many language areas due to shared culture, each one will adapt it to their own language. John was originally a Hebrew name "Yohanan". As it spread through Christian countries and areas influenced by them, it became Giovanni, Jan, Eoin, Juan, Seán, and John in English, and many more. We're not sticking to the original pronunciation any more than anyone else. And so it's the same with Paul/Pól.
Because Irish, just like English, distinguishes between a simple present tense (Paul drinks water - Ólann Pól uisce) and a continuous present tense (Paul is drinking water - Tá Pól ag ól uisce). The two forms have different meanings in both English and Irish.
(Many other European languages do not make this distinction).