It used to be a pronunciation issue: a girl would be formally named
Marian, or indeed
Marion, but in the local dialect her name would sound more like Mirren. In one of
John Buchan's Scottish novels this is made very clear. Similarly,
Jane (
Janey),
Jean (
Jeannie) or
Janet would be pronounced like
Jinny or Jinty for everyday use.
St Mirren football club claims to be named after St Mirren or Mirin, but he is a very mysterious and possibly mythical character.
As for it's renewed popularity, could this be a result of modern parents giving their children the (version of ) the name they will call them by, rather than a supposedly correct version that will never be used? Like naming a girl
Sally or
Megan instead of
Sarah or
Margaret?