Rare indeed! I can find no reference to a 19th century Scottish architect named James Dairon, which is not to deny his existence, just to note his low profile. The name looks French, but neither of my French sources, Dauzat's dictionary and Jean Tosti's website, has a notice of the name. Nor, for that matter, do works dealing with British surnames, including Black's "Surnames of Scotland". I have found a minor French place name, Airon-Saint-Vaast, Pas de Calais, which MIGHT have given the surname, D'Airon -> Dairon, but no evidence of that.