Mr Sargent conceded he went too far when he used headed notepaper from the Department of Agriculture to influence the work of this investigator.
The garda had brought charges against Dominic McGowan, aged 39, of 14 Cardy Rock Close, and his neighbour Stephen Mulvany, aged 34, of 12 Cardy Rock Close, for fighting with each other.
Mr Sargent said he had spoken to Mr McGowan on a number of occasions and felt obliged to advocate on his behalf because he felt he was the innocent party.
In March 2009, Balbriggan District Court found both men shared part of the responsibility.
Mr McGowan was fined €500 while Mr Mulvany was jailed for four months with a fine.
Mr Sargent said in spite of this, he did not believe Mr McGowan should have faced charges for standing up for what he believed was right.
“My only motivation was to help an individual who felt very vulnerable, was afraid for his own safety and simply wanted to see justice be done.
“I now realise that being as intent on helping that person as I was I actually overstepped the line. And of course I regret that.”
Mr Sargent intervened by way of letter.
It was written to the investigating garda because he wanted to keep it low key and copied to Mr McGowan on the same day, June 11, 2008.
This was 15 days before Mr McGowan was due to appear before Balbriggan District Court to face charges of threatening and abusive behaviour.
Mr Sargent’s single page letter to the garda said he was shocked Mr McGowan had been called to answer the charges.
It asked the garda to consider interviewing witnesses who had not been spoken to. He said it was his understanding none of Mr McGowan’s witnesses had been spoken to and it was “wholly inappropriate” to proceed with the summons on this basis.
He told Mr McGowan if this was not successful he would go further.
The incident happened on September 6, 2007, in the neighbourhood of Cardy Rock Close on the northside of the town.
According to Mr Sargent, Mr McGowan was prompted to act after seeing a signpost being removed by some children.
Mr Mulvany had a previous conviction and has appealed the latest sentence.
Last night, Mr Sargent said Mr McGowan was doing the right thing and should have been applauded instead of facing criminal charges.
“A young man who sees damage being done to public property tries to take responsible action by mentioning it to a parent.
“And instead of getting any level of appreciation for going out of the way in the way that he did, he was hospitalised, attacked and some months later was very frustrated.
“Not only at the ongoing pain and suffering brought about by the attack but also by the way he felt the witnesses, he felt should have been called, were not being called.”