Question

what is ironic about montag letting a sleep - lozenge dissolve on his tongue?

Explanation:

In "Fahrenheit 451", Montag uses a sleep - lozenge, but he is avoiding facing reality (like his wife's overdose). The irony is that a sleep - lozenge, meant to help sleep, is used to evade waking up to real issues, contrasting its intended use with his avoidance of truth.

Answer:

The irony is that the sleep - lozenge, meant to aid sleep, is used by Montag to avoid confronting the harsh reality (like his wife's overdose and the emptiness of their lives), contrasting its intended purpose with his act of evading waking up to real - life problems.