I have searched all of County Cork and all of the republic of Ireland for a Thomas McVay and found none in birth/baptismal records with that spelling of the surname. I have found 13 Thomas McVeigh. McVeagh, McVey and other versions of the name in all other counties but none in Cork
I have also searched all of Northern Ireland and the republic for a marriage of Thomas McVay and found 88 marriages all with the same variety of spellings but again none from Cork. Using the same search engine, I looked for McFairling and got no hits. The spelling you got for her first name (Francis) is the male version of the name, Frances is the female version.
Can you provide me with any indication of the approximate era you are looking for or expecting to find them in. Your message was in the Fermoy Message board, is there any reason why you placed it there or was it an error. Have you any indication of where they came from in Ireland. The vast majority of the search hits were in Northern Ireland and the two surnames would even at present be much more closely associated with Northern Ireland than the Republic.
My apologies for the delay in a response to your message but it was only today that it was brought to my attention.