Situational, because Jim sold his watch for combs to give Della. The situational irony part comes when Della sold her hair to give Jim new straps for his watch. They both gave their most prized possessions to add more value to their lover's possession not knowing that they did the same thing.
Notice how the word soliloquy is a bit like the word 'solo'. This is basically what it is: a solo in drama, where somebody is projecting their inner thoughts. This gives the audience direct access to the character's feelings.
