The irony contained in the prayer is present in the description of the adjectives that are affected to Elizabeth. As she is described as a cold woman, it is ironic and contradictory that she is cold sniveling woman, a characteristic that is attached to a <em>sensitive and fragile person</em>, thus generating descriptive conflict and including the irony of the phrase.
In this poem, Death is personified. Instead of talking about death as a thing, or even an event, death becomes a person.
More specifically, Death is a gentleman caller who comes to pick up Dickinson, almost as if to take her on a date. She talks about his kindness and "civility," emphasizing his gentlemanly nature. Death takes her on a carriage ride into eternity.
The impact of this use of personification is to change death into something scary and unknown into something honorable that visits everyone.
It is an adverb because because it’s an expression