I am a Ireland XO volunteer for Tiernascragh. The Mogan name does not ring any bells for me locally - although I will see if two other local volunteers in both Tiernascragh know more than I. Will ping them separately. Mogan is not a local name to these parts. It is more a North and West Galway name. You will find no Mogan name in these areas for the Griffiths Valuation or the 1901 / 1911 census - just double checked online. A blacksmith of course was a migratory profession. I am aware of a blacksmith family who came to Tiernascragh around these times - maybe there was a vacancy caused by a departure. Have you found the naturalisation records of Michael or Catherine Mogan or death cert/Obituary - they are usually good sources of local origin in Ireland. If you find other people descendant, you can sometimes come across handed down written letters. Was there any reference that led you to Galway bar the Church Records?
I'll drop a note to Pat Madden (Tiernascragh) and Ioma Whelan (Killiomor) to see if they can help.