The dead man, who was from England, downed between eight and 10 spirits in a half-pint glass while showcasing his party trick and racing against a work mate drinking a half-pint of lager.
In the first case under the new liquor liability law, prosecutors said barman Aidan Dalton, aged 26, was given the okay by bar manager Gary Wright, aged 34, to serve the cocktail to a customer who had been drinking heavily for hours.
Graham Parish, from Calder Terrace, Lomeshaye village near Nelson, East Lancashire, collapsed after the drinking competition and was taken to an upstairs conference room by his friends. When he was checked at 6am the next morning, he was dead.
He had booked into the hotel on June 30, 2008, while he was in Thurles to do work at the Dew Valley meat plant.
Mr Parish arrived at the bar at about 6.30pm and started drinking pints of Guinness. Five other English workers joined him to celebrate his 28th birthday.
At about 10pm, he put a shot into his pint and drank it. Unknown to him two more vodkas were put into his pint, which he drank.
A drinking competition started and Mr Parish said he could down 10 spirits faster than the others could down a pint or half-pint.
On the opening day of the trial at Nenagh Circuit Court, the jury was told barman Aidan Dalton asked the bar manger Gary Wright if it was okay to give him the shots and the bar manager said it was.
Both the accused men, who have addresses at Kilfithmone, Borrisoleigh, have pleaded not guilty.