Because the vocative case in Irish relies on Irish spelling rules, it has long (as in centuries) been the practice to use Irish forms of traditional names (Biblical names and other traditional English names) in Irish.
De Bhaldraithe's 1959 English Irish Dictionary includes this paragraph in the Plan of the Dictionary:
Proper names are included in the body of the dictionary and not in separate lists. Personal names which have the same form in Irish and English have been omitted. It is a well-established practice to equate English and Irish personal names which have no historical relation to each other. These equations are recorded here. The problem of whether to use an Irish form or the original form of a foreign name is an extremely difficult one. The aim here has been to record Irish forms, but not to proscribe the use of the original form.
The EID has an entry for "Paul".
I agree. I put "Itheann Paul" and it wasn't accepted, which it should hav been. Particularly since i had previously had my translations into English rejected when i did Anglicize names, so it was natural for me to think i should avoid translating names in order to get my answer accepted.