Per Lancs BMD it looks like Mary Kelly's maiden name was McKey or McKay. There's a possible marriage in Preston in 1854: John Kelly/ Mary MacKay (Preston Register Office or Registrar Attended).
how did you figure out thats the right Mary? I only ask because i can't find any records that link "Mary Kelly" to specifically "Mary McKey/Mckay/Mackay" vs any number of "Mary so and so's" that John could have married.