Having looked in Dwelly's dictionary it looks as if the genitive of "sioda" is the same as the nominative.
    Seeing a sign saying "Tobar Mhuire" on the road into Tobermory (St. Mary's Well), I mistakenly thoght "Mhuire" was the genitive of "Mairi". However, the dictionary says that Muire is the formal equivalent of Mary, and that Mairi is the colloquial form. Maybe nowadays Mairi is more common and Muire tends to be reserved for St. Mary.
   Therefore I now think the song title is probably "Co\ta Sioda Mairi Bha\n".