Miguel Saez Sanchez (32) has claimed his dismissal was "disproportionate" after he unwittingly consumed the cookie at a party in Los Angeles.
He had also taken a slimming pill, which are specifically banned in the Aer Lingus cabin crew manual.
An Employment Appeals Tribunal heard Mr Sanchez, of The Way, Dunboyne, Co Meath, thought passengers were talking about him during the Aer Lingus flight from San Francisco to Dublin on March 20, 2009.
He also believed an Aer Lingus "plant" was taking his photograph and thought he was being attacked by fleas under his skin during the flight.
Lawyers for Aer Lingus claimed Mr Sanchez' behaviour constituted gross misconduct and they were right to dismiss him.
Representatives for Aer Lingus said the company could never permit a cabin crew member to fly with them while under the influence, given the nature of the job.
Mr Sanchez started working for Aer Lingus in November 2006 and his employment ended on April 29, 2009.
Tom Mallon, counsel for Aer Lingus, told the tribunal chaire by barrister Mark O'Connell that Mr Sanchez was due to work on a flight from San Francisco on March 20, 2009.
Mr Mallon said Mr Sanchez was at a party in Los Angeles the night before his return trip and took a cannabis cookie.
He turned up for work on March 20, 2009, and did not advise his supervisor he was unwell until the flight had taken off.
Mr Mallon said Mr Sanchez was obviously suffering from paranoia, and was unable to perform his cabin crew duties. He was told to "stand down", and he slept for the rest of the flight.
He also said that during an internal investigation by Aer Lingus Mr Sanchez did not deny the essential facts.
Captain Malachy O'Curry, for Aer Lingus, said that given the nature of the job it is absolutely crucial that all cabin crew present medically and physically fit for work.
Cabin crew manager Mary Montgomery claimed Mr Sanchez got on a flight knowing he was unfit for work. She said that due to the severity of the incident, and the safety implications, she had no option but to dismiss Mr Sanchez.
Ms Montgomery, who was a cabin crew member for 21 years, also said she would not fly again with Mr Sanchez as the "trust is broken".
Cathy McGrady, counsel for Mr Sanchez, said he initially thought he was fit for work, but when he realised he was unwell he immediately brought it to the attention of his supervisor.
Ms McGrady said he will contend his dismissal by Aer Lingus was unfair and disproportionate to the incident. Tribunal chairman Mark O'Connell adjourned the matter to next April.