A Medical Council inquiry has heard that Dr McKenna was training for an Ironman triathlon at the time and was in a bad mood as a result.
His barrister, Marguerite Bolger told a fitness to practise inquiry her client's attitude was less than satisfactory.
"At the time he was engaged in very, very high levels of training as an elite athlete. He described it as being one of his two passions in life.
"After it happened he realised that there was only room in his life for one of those passions and he has turned his back on his athletic career."
Dr McKenna yesterday consented to being censured by the Medical Council and gave an undertaking never to repeat the behaviour complained of.
The 28-year-old, who is originally from Co Sligo, faced five allegations of professional misconduct and poor professional performance from his time as a junior doctor in St Michael's Hospital, Dun Laoghaire between January and April 2011.
Among the other allegations against Dr McKenna were that he refused to 'rechart drugs' required for a patient who was re-admitted through A&E after being discharged the previous day and that he prescribed the blood-thinning agent Warfarin to a patient without first obtaining laboratory blood test results.
Dr McKenna also admitted to falsifying the results of a blood test for a patient he was referring to the Warfarin clinic.
He told the inquiry that at the time he believed the "tactical falsification" of test results was common and that he got this impression from listening to "canteen talk".
Dr McKenna told the hearing he did not enjoy his job at the time, "you do a lot of secretarial work and a lot of nursing work, you don't really feel like a doctor. In hindsight it was a job I wasn't suited to".
He said being brought before the Medical Council was the single worst thing that's ever happened in his life.
Dr McKenna said he was now specialising in paediatric emergency medicine in Sligo.
He added that he sees himself as being suited to his discipline. "I potentially would like to see myself as a consultant in emergency medicine."
Ms Bolger said her client was determined to become a better doctor. "Dr McKenna for all his hysteric bravado, for his poor attitude, for his appalling interpersonal skills, has learned a huge amount from this process."