Answer:
There is no such language as Celtic. The word "Celtic" refers to a group of more than a dozen different languages, 6 of which are still spoken today:
Irish: mo
Scottish Gaelic: mo
Manx:
Welsh: fy
Breton:
Cornish:
In Celtic languages "my" usually changes the first letter of the next word;
in Irish "my son" would be "mo mhac"; mac being the unmodified root word.
This would not apply if the first letter is a vowel.