The irony in "Misery" is all contained in the fact that the simple old man imagines he can communicate his misery with a dumb animal. Irony is usually something that would be funny if it were not so painful or pathetic. Some people might laugh at him if they saw him. No doubt Potapov does get some temporary relief from talking to his horse, which seems to be the only friend he has left in the world.
In Macbeth, Malcolm tells Macduff, who has just learned of the slaughter of his wife and children:
Give sorrow words. The grief that does not speak
Whispers the o'erfraught heart and bids it break. IV.3