Answer:
In most of the diverse critical uses of the term "irony" there is a root sense of dissimulation or of a difference between what is asserted and what the real case is.
in this poem these is an irony on the usage of the word Death.
Here death is personified as a sober suitor ho comes to release his wife from the boredom of life. He is said to have brought Immortality as their chaperon . Death being the cessation of life is regarded to be a gateway to eternity.The poet Emily Dickinson appears to attain immortality through death. This is in contrast to the real belief of people in death .This also shows the robust optimism of the poet. She denies the real case.