Fire is the symbol of life.  There is a quote referring to the burning of the "turf fire", meaning the home fire.  This fire gives light and warmth to the family.  It is a living source that consumes and carries on, as the living people are supposed to do.  And, like the bird the phoenix which is said to rise from its own ashes, it is a symbol of rebirth.  Gabriel is reborn in this story.  The dinner party, the talk of Micheal, all of it makes him realize he has not been living a full life.  He has been a spark with no flame.  The suggestion of Joyce is that he will move on from the timeline of the story to better enjoy and experience his life.