John's heroic efforts, first revealed by the Herald, resulted in him being present with an award by a prominent animal charity.
Kearney (20), who has a string of criminal convictions, was sentenced by Dublin District Court.
And speaking to the Herald today, John described the sentence as "complete justice... I wasn't sure he wasn't going to be handed a prison sentence but it shows that the judge does not tolerate cruelty to animals.
"I was in court and I am absolutely delighted by the decision. Barney could have died that day. It was an extremely upsetting incident for me."
John was sitting on O'Connell Bridge in the evening of July 3, 2011 when Kearney grabbed Barney from his lap. Passer-bys watched on in shock as John courageously swam through the heavy current to save the pet from drowning. Both were subsequently rescued by members of Dublin Fire Brigade.
Kearney, who is from Crumlin but has an address at Conyngham Road, in Dublin city-centre, was later charged under the Protection of Animals Act in connection with the incident.
Last week at Dublin District Court, his solicitor had indicated that Kearney, who already has 138 prior criminal convictions, would be admitting that he had committed the crime.
Yesterday, the case resumed and Mr Byrne came to court, without his rabbit, to see Kearney sentenced.
In pleas for leniency, defence solicitor Maura Kiely told Judge Smyth that Kearney, is currently serving a sentence and will not be released until April 2015.
Meanwhile, John is still living on the streets.
"I'll be homeless again this Christmas, with Barney and my pet dog Lilly. Things are really difficult and I'm dreading the cold weather setting in. I'm hoping someone will show me some good will and give me a helping hand," he said.