O'Donnell-Nelson look up please

[kandddad]

Looking for any information regarding JOHN O'DONNELL (b. abt 1831, d. Jul 27, 1896 in Ballybeg, Co Donegal) and wife, BRIDGET NELSON (b. abt 1824, d. Jan 19, 1920 in Altashne, Co. Donegal).  I'm told, they were married, in Co Donegal, Feb 22, 1856.  Carndonagh has been a constant family mention.

My great grandfather was their son, George Lawrence O'Donnell.  He reportedly was born in 1864 and immigrated to the US in 1884 (can't find record of his arrival in the US).  I'd be very interested in names / married names of my great grandfather's siblings (with birthdates/spouse names, or whatever information may be available).  Or, if available, the names of John and Bridget's parents/siblings.

Any assistance would be welcomed.