Asked if they knew that Niall had carried a donor card, his mother Veronica told the Herald: "That's just what Niall would have done".
Niall was just 18 when he was set upon while walking down the street in an unprovoked attack on October 13, 2010.
He had his head stamped on and repeatedly banged off the ground and against a wheel of a parked car.
He was rushed to hospital before he was transferred to Beaumont Hospital, but died the following day from massive brain injuries.
However, he had signed a donor card three months earlier and his liver, pancreas, kidneys and lungs were used to save the lives of five people.
Douglas Ward (36), of Loughantarve, Knockbridge, Co Louth, pleaded guilty to Niall's manslaughter and will be sentenced tomorrow.
Niall's family have told of how they have received letters from each of the recipients of his organs to explain the difference the kind act has made to their lives.
Speaking of Niall's generous nature, they said in their victim impact statement that reading the letters from the people Niall had saved was "heart-wrenching" yet at the same time "consoling".
"We are grateful for the letters which we received from each recipient just to let us know that they are all doing fine and that their lives have been changed for the better because of Niall," his dad David said.
A teenage recipient wrote to Niall's family and said the operation had transformed his life and that of his family.
Another referred to Niall as an angel, assuring his grieving family that he would live life to the full.
"This is what Niall would have been doing if only he had been given the chance," he said.
"As Niall was so young and healthy we pray that his organs will last for years.
"And we now know that a lot of people signed up to be organ donors as a direct result of what Niall did," he added.
On the morning of the fatal assault Niall had attended an interview for the Irish Army.
Evidence was heard at the Central Criminal Court that Ward had been drinking all day and was with four men and two women who were trying to get into a house party at Castle Road when Niall and his two friends were walking by making their way home.
A fight developed and during the row Niall became isolated with Ward and suffered the savage beating.
A witness told gardai she saw Ward "bouncing" Niall's head off the road up to 10 times and kicking him in the head a number of times.
Another witness gave a graphic account to gardai saying it was a "continuous beating with vicious kicking and punching."
A third witness said he saw a "fella on the ground who looked like he was lifeless and a man kicking and stomping on his head."
He said Ward was "banging" Mr Dorr's head off the wheel of a parked car.
Ward constantly denied he was at the scene.
After the incident, Ward and another male got a taxi to an off-licence where they bought more alcohol and went back to his house to continue drinking.
He will be sentenced at the Central Criminal Court tomorrow.