The Tyrones, in Act 2, Scene 2, share a very tense lunch. No one says specifically that Mary has relapsed, but everyone knows it, and there is great silence and tension at the table. A phone call comes in, which Tyrone goes to pick up, saying it's from "McGuire." Mary chimes in on the subject:
Mary:
Indifferently.
McGuire. He must have another piece of property on his list that no one would think of buying except your father. It doesn't matter any more, but it's always seemed to me your father could afford to keep on buying property but never to give me a home.
McGuire forms a sort of foil to Tyrone. For one, he is the cause of Tyrone's financial trouble, as all his money is tied up in property sold to him by McGuire. Thus, McGuire's presence in the story serves to elaborate and create contrast within Tyrone's character; it is only through McGuire's financial antagonism that the reader sees Tyrone in his impoverished state. McGuire's job of falsely hawking land forms an interesting opposite to Tyrone's job of acting; both have a career of lying that conflicts with the other.
Note that Jamie also accuses Tyrone of treating Edmund worse than one of his plots of land. McGuire comes up again in Act 4, after Edmund figures out that he's been sent to a state sanatorium and that the reason he can't afford it is because (says Edmund) "you went to the Club to meet McGuire and let him stick you with another bum piece of property!" McGuire can be seen, then, as a sort of shadowy foil, never seen on stage, but nonetheless the inevitable cause behind the obsession with land that dooms his relationship with his family. McGuire is the human incarnation, in the play, of Tyrone's ugly selfishness and miserliness. He is the foil that prevents Tyrone's attempts to keep his family happy and healthy.