Dunner

I have included Dunner as an O’Donoghue name variant on phonetic grounds after I found the name in the Barbados phone book. Donough is a true variant and the names sound the same. However I cannot say categorically that it is, but the fact that your grandfather came from Limerick certainly helps. However it seems to me that the name might also be derived from the German Donner and as there were immigrants from the Palatine region of the Rhine who settled in Limerick that becomes a real possibility

Source #1:
Barbados phone book
Source #2:
Source #3:
Source #4:
Source #5: