Gaelic and Celtic are not separate languages as such. Rather, the Gaelic languages are a sub-family of the Celtic language family. So it is unlikely that a name would mean one thing in Gaelic and something else in 'Celtic'.
I think the 'peace' meaning is probably something fabricated by "baby name" books and sites to give the name a more interesting meaning than 'Ireland', particularly if the false meaning dates back to the time when the Irish were not well-thought-of in English-origin society.
♦ Chrisell ♦
All we have to decide is what to do with the time that is given us. - J.R.R. Tolkien.