Originally Posted by Peadar
There's no copping onto anything.
The passport office identify you from your birth cert and assign you a "Holder" number. Regardless of how you identify yourself, you'll always have the same holder number and therefore they'll always know who you are.
It's not uncommon in Ireland for a father and son to have the same name. In many cases, the son would be known by another name such as his middle name. That name could appear as his name on his passport.
Changing your name to Irish is not unlawful or a problem for the Passport office. They're far too busy dealing with actual fraud.