A correspondent [ID] has an 19th century ancestor in Scourie, Scotland whose baptismal forename was Meran. She signed herself Merran. However she was recorded on Census returns as Marion or Marrion. It seems very likely that this is another variant on Marion/Marian.
Another correspondent [RB] has an ancestor registered in Shetland, Scotland as Meran/Merran. One parish entry gives "Mary Ann" as the spelling so this is another possible derivation of the name in Scotland.
The spelling Merryn has also been found by the same correspondent [RB] in Sutherland, Scotland. This is the spelling of St Merryn, a Cornish male saint, but no connection is known.