I have been searching for any marriage between Michael McGrath and Johanna Sheehan in Mitchelstown, Cork, based on the birth certificate for my great grandfather (which I will post next). I performed a 20 year search on IFHF for both names, then matched up the years and had 9 to chose from. After paying for 8 I got 'Johanna Sheehy' so at some point, either back in 1874 or in recent transcription, there has been human error.