Edward Robert Manning/Turnbull

I am searching for any information on my x2 GG Edward Robert.  His surname could be either Manning or Turnbull.  He seems to go between the two!
I have found of him on the 1901 Irish Census as Edward Manning where it states he was born in Co. Waterford in 1854 and the 1911 Census again as Manning where it states he was born in Mothel, Co. Waterford with his year of birth still at 1854.
At the time of the Census, he was living in Dublin City.   His son Robert was born in Dublin in 1879 under the name Robert Edward Turnbull.  His son also goes between the two surnames.  Manning on the Census but Turnbull on his marriage certificate and for most of his children births.  Both Edward and Robert are buried as Mannings.  I have also found Edward on various Dublin trade directories as Edward Manning.
 It’s a mystery why, my only theory could be that perhaps Edwards mother was a Manning and his father a Turnbull or vice-versa.

Im after any information on Edward.  Im hoping a birth reference from Mothel could help and would be fantastic, hopefully it may reveal the name of his parents or other family members.  If anyone could help I would be most grateful.

Cheers

Carolyn, Melbourne, Australia