The baptismal records for Moyrus/Carna RC parish start in December 1853 and marriage records start in September 1852. https://registers.nli.ie/parishes/1054 This means that no baptismal record exists for Mary Folan. I saw a transcription of the 1866 marriage record but I could not find a civil marriage record.
You mention Letterard townland. The 1855 Griffiths Valuation head of household listing for Moyrus civil parish shows five Folans living in Letterard-- John, Mary, Patrick Thomas and William. Do you know which of the four males was the father of Mary?
Depending on the name you provide, we can look for death records and if we get lucky, we may find the correct record with the spouse as the informant. (The fre images from 1871 on.e site www.irishgenealogy.ie has actual death record
Let me know.