I wasn't sure if you ever found the McMahon family you were in search of, but I did come across the name Oliver while researching my McMahon family in Meath records. However, the only one registered in all of Meath wasn't born to John McMahon. The records says Oliver, born 13 Mar 1834 to Edward and Bridget MacMahon in Kells, Meath, witnessed by Mark Tully and Mary Fitzsimons.
Hope this helps, even though it has been a few years. The name Oliver is fairly uncommon, given that most McMahons were Catholic and the name Oliver would probably date back to the time of Oliver Cromwell, who sought to squash the Catholic religion and the McMahons of the Oreil.
Slightly off topic, I was wondering if you found out where your McMahons had lived in Meath, specifically, if there is a townland associated with them? My gg grandparents, Laurence McMahon and Bridget Dolan remained in Ireland long after their families had departed for Jersey City, NJ. My great grandma, Mary McMahon married Patrick Lynch and moved from Meath to Cavan, and later (about 1920-25) many of their children left Ireland for New Jersey, settling in Bayonne. If I am correct, several of my family members were sponsored by or temporarily stayed with the McMahons of Jersey City upon their arrival in the US. I am thinking that our families might be distantly related?
In the early 1850's my greatx2 grandfather, Bernard John immigrated with his family, mother (possibly Mary), father (possibly John), 4 brother's, Michael, Oliver, Edward, Patrick and a sister(unknown). They may be from Moynalty. Can you help?