Originally written to take place in 1982 when E.T. the Extra-Terrestrial (1982) first came out, but budget cuts obligated it to be set in 2002 when it was made. Coincidentally, the 20th anniversary edition of E.T. played in many cinemas in 2002, so that plot point didn't have to be changed.
Jim Sheridan said in interview that the character of Frankie, the dead son, was based on Frankie Sheridan, his late brother. The film is dedicated to this Frankie in the end credits.
During casting calls, director Jim Sheridan recalls that after a long day of auditions, he was very glad to have found Emma Bolger to play Ariel. He said that that little girl "told" him that he had to cast her sister Sarah Bolger as Christy. Sarah was only 10, and he had wanted Christy to be 12 or 13. After exhausting all other options, he auditioned Sarah for the part of Christy and she turned out to be so impressive that he didn't mind about her age.
The child actors (Sarah Bolger and Emma Bolger) called "cut" and "action" in every scene. This was decided by director Jim Sheridan and the girls early during filming, so that it would make the acting easier for them.
Samantha Morton, who plays Sarah, was 25 years old when the movie was filmed. She is only 14 years older than Sarah Bolger, who plays her eldest daughter, Christy.