I'm glad you came to this board! You've got me focused on the O' Donnell's again after some time out!
I never looked at Ardfinnan, as you say its not too far away. Had a quick look and there are quiet a few O' Donnell records. I don't see Edmond O' Donnell born 1861 though. You seam convinced Catherine is a sibling of Edmond and William even though her father is stated as dead by 1889. On Edmond marriage cert 23-Feb-1895, it does not mention his father as being dead. Was it the NORM to state if either the grooms or brides father was dead at time of marriage, or could some slip through the net? I did not locate any birth certs for either Edmond or William, you say Edmond's mother was Ellen Wall, do you know Catherine's mother?
Wait a minute, I just looked at marriage cert of Catherine again, could the transcriber have inserted it wrongly into the computer. Catherine is in the husband column and Patrick is in the Wife column. It says "wife's father dead" maybe it should be 'husbands father dead'. Long shot?
William called his fourth child Ellen (but he married an Ellen too)
Children
Mary
Catherine
Margaret
Ellen
Bridget
Annie
Andrew Christopher
Sorry, that was a typo,
Andrew O' Donnell died 8-Sep-1828 aged 68 from Clerihan, buried in old Clerihan graveyard. His wife was, Eleanor Slatery died 18-Sep-1833, aged 67.