Hi Brien,
I am from County Meath, and lived in the Nobber area for a number of years but for all my adult life have been living in Dublin. However chasing up my own family roots have meant often dealing with Nobber RC Records among others. You will find a Microfilm of the Nobber RC church old record books in the National Library in Kildare Street, available free for you to search through. Its Reference number is P4183. It covers Births , Marriages and Deaths from approximately 1755 to 1865. The writing is sometimes very faint and hard to read, and I think some pages are missing but I had a quick half hour looking for your McMahon family and I can say I found their Marriage record on May 14th 1838 , Sponsors Tim Farrelly and Patt McMahon !! I also saw births of some of the children, Michael on March 20th 1839, Thomas on Dec 18th 1848 and Catherine on Aug 30th 1851. When you come on your visit you will be able to look longer and more closely and you may well find the others as well, perhaps in the adjoining years as they are often a bit inaccurate.
As for tracking back any farther you should know that "Farrelly" and especially "McMahon" are fairly widespread names in that area and it will be difficult to which one is the ancestor.
As for spellings, you are also entering a time when in that area a large percentage of the RC population in particular were Irish speaking and whenever some clerk had to write down their name he would write down what he thought it should be spelt as in English. Hence the various spellings you will come across.
I wish you luck with your search, and if I can be any further help I will be delighted to assist.