Possible Clues:

• Tonality indicator
• Block of flats, for example?
• Initials on item used for entering a couple of flats, possibly?
• Critical mark in block of flats, perhaps
• Notation after a clef
• Connolly was one in Kildare yard with Mark
• Staff notation
• Art genius baffled by important musical reference
• What new tenant receives, and gives, for one flat, say
• 'John Hancock' on the Declaration of Independence?