The mother of Alex Cuthbert said she would "never forgive" PJ Carey Contractors Ltd for breaching safety regulations.
Speaking about the day her four-year-old son was killed, Ms Landy said she was standing on her balcony when she thought she saw one of the children fall and heard someone shout his name.
When she noticed a crowd gathering on the road, she shouted out if it was her "young fella". She passed a friend on a corridor and asked the woman if her son was okay.
When the woman just shook her head, Ms Landy ran down the six flights of steps in her bare feet on to the road outside, and saw Alex lying on his right-hand side with blood everywhere.
"His eyes were open but when I found he had no heartbeat, I laid a jacket over him."
Ms Landy said she was grateful for the support she has been given by the community in Ballymun. It raised funds to erect a memorial in the shape of a teddy bear which had Alex's photograph on it, and it was placed on the site where "he took his last breath".
The Health and Safety Authority (HSA) welcomed the court's decision, and said it would act as a warning to construction companies to implement and monitor safety policies.
Senior HSA inspector Jim Heffernan said: "An important lesson from today's case is that safety procedures should be actively implemented and constantly monitored to ensure that construction work being carried out near public areas take account of all possible risks.
"Having safety policies in place is one thing, but actively implementing them is another."
Another spokesman said: "This sends out a warning to all firms to police their safety procedures. We welcome the judge's decision and his remarks."