Situational irony occurs as the poem opens. The speaker "could not stop for Death." She presumably was busy with many things to accomplish, and she simply didn't have time to quit the tasks which seemed of paramount importance from an earthly perspective. Nevertheless, Death (personified) stopped by and picked her up anyway, transporting her away from the life and the busyness she was so consumed with. What she expected to happen (continuing on with her planned life) is quite different than what actually happened (those plans came to an abrupt end when she died). This is situational irony.
It's also rather ironic that Death is personified with actions that demonstrate kindness. Typically, a representation of death is conveyed with symbolic darkness. Death is often portrayed using a threatening or ominous mood, ripping unwilling humans away from their earthly lives. It's ironic that in this poem, the speaker goes willingly with Death, even though her plans have been disrupted. She conveys no sense of fear or reservation about departing with this unknown host and climbs into his slow-moving carriage with a complete acceptance of her fate. She passes various metaphorical representations of her own life with a sense of calm, and instead of resisting her date with Death, she meets the moment with grace and resolve.