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I checked the 1911 census for Co. Waterford http://www.census.nationalarchives.ie/ and there were no McAvoy, MacAvoy or Mackavoy people in the county. There was one Mackavoy in the entire country in County Clare.
I would suggest that you add your great great great gandfather's story to our XO Chronicles site and possibly someone will see a connection. https://www.irelandxo.com/ireland-xo/history-and-genealogy/ancestor-data...
You may want to consider autosomal DNA testing. Possibly you will find distant cousins with McAvoy connections.