Margaret dies as an infant, while the McCourt family is living in Brooklyn.  Frank McCourt portrays this death as being the catalyst for the family returning to Ireland; life in Brooklyn is just too hard.  Also, he shows the devastating effect of the death on his mother, who is too distraught to even see after the care of her remaining children.  As McCourt portrays it, she never quite recovers from her grief. 

One way to interpret Margaret's death is in the portrayal of the nuclear family.  Not only Margaret, but also the twins, die young, at least in part as a result of the poverty in which the family lives.  McCourt suggests throughout his memoir that the attempt to keep the family together is a false goal.  It is when the boys leave the home that they begin to thrive.