Thomas Ernie McDonagh Snr, from Hazelbrook in the town, had denied being involved in an incident on February 17, 2012, in which slash hooks, nail-studded baseball bats and chair legs were used to batter Dermot Kildea.
Mr Kildea was knocked unconscious during the attack at a house off Long Lane and rushed to Letterkenny General Hospital where he had to undergo emergency surgery.
He said he had remembered seeing Mr McDonagh just before the attack but couldn’t remember who had hit him.
He later identified two men from photographs but not Mr McDonagh.
An eye-witness to the incident said she saw Mr McDonagh with the crowd but couldn’t say for certain she saw him throw any blows, adding: “Half of them were at him.”
Another witness, now living in Falcarragh, who gave a statement to Gardai the day after the attack said she could no longer remember the incident.
A jury took just 20 minutes to find Mr McDonagh not guilty of assault causing harm.
Judge John O’Hagan praised the jury for their patience in dealing with the case which was interrupted on numerous occasions over legal arguments.