I scoured the newly indexed Ireland Catholic Parish records on both Ancestry and Findmypast for my supposedly Irish Marmon ancestor. Cross-checking the files from both companies I found all 44 births matched - the same on both. Could they have cooperated on the indexing?
Ancestry's index includes an additional 8 marriages and 5 burials/deaths not yet indexed by Findmypast. Some of them looked more like Marmion than Marmon to me even though I specified an exact match.
The graph at the left shows the relative frequency of top Irish surnames in 1890, as identified in Varieties and synonymes of surnames and Christian names in Ireland and reproduced in Table 1 from A Survey of Irish Surnames 1992-97, by Sean J Murphy.
The blue line is for the 1890 frequency, the grey from the Ancestry Catholic parish record index. Murphy and Ryan stand out as of higher frequency in the Catholic record, Smith, O'Brien and O'Neill of lower Catholic frequency. In 1890 Smith and O'Neill are more heavily concentrated in Ulster, Murphy and Ryan in Munster which is where O'Brien is also well represented.