An internal investigation had been launched after the school learned he was running what it considered a rival grinds business.
The former principal of Leinster Senior College in Newbridge has lost a legal challenge to his dismissal.
Karl Hegarty had taken a case under the Unfair Dismissals Act after he was sacked for gross misconduct in January 2023.
An internal investigation had been launched after the school learned he was running what it considered a rival grinds business.
Mr Hegarty, according to the Irish Independent, claimed he had verbal permission from his employer and said he had noted this in his work diaries.
However, the Workplace Relations Commission heard that he shredded those diaries after being asked to produce them.
The WRC adjudicator said the destruction of potential evidence "entirely undermined" his credibility.
It also claimed that Mr Hegarty failed to give clear information about his income from the business, and that he had been aware of his obligations, even if he hadn’t signed a contract with a non-compete clause.
His complaint was formally dismissed in a decision published yesterday.

New TII Report Confirms Congestion Is The Main Factor Influencing Risk Of Collisions On M7/M9 Merge
Concerned Response From Parents Regarding The Hot School Meal Scheme
Kerry's Jessie Buckley Makes History At Last Nights Academy Awards In LA
Man Seriously Injured In Overnight Assault In Naas
Kildare Drugs Arrest
22 People Arrested For Driving Under The Influence Since Yesterday
Health Minister Meeting With Kill-Based Contractors BAM, Amidst Concerns Of Further Delays To New Children's Hospital
Celbridge Facing 300+ Year Wait For New Homes Based On Current Building Rates