Let us remember that irony is defined as the gap between appearance and reality and how it is exploited. There is an excellent example of situational irony at the beginning of this scene when Amanda comes back into the appartment and surprises Laura when she is polishing and cleaning her glass menagerie. Note Laura's reaction when she greets her mother:
Hello, Mother, I was-- (She makes a nervous gesture toward the chart on the wall. AMANDA leans against the shut door and stared at LAURA with a martyred look.)
What is ironic about this section of the play is that Laura pretends to be busy working at her typewriting course, when she has given that up months ago. However, Laura doesn't know that such gestures are completely unnecessary because her mother has just gone to her college where Laura is supposedly studying and found out the truth. Laura cannot pretend to be working hard on her studies any more.