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The Alcohol Regulatory and Licensing Authority (Arla) has suspended the on-licence for Bransons Bar in St Andrew St after alcohol was sold to an underage girl during a controlled purchase operation in August last year.
The bar's manager Jeffrey Smith also had his managers' certificate suspended for three weeks.
Mr Smith agreed a mistake had been made by a now former employee but since then he has made changes to help staff more easily determine if a person was 18.
As the 17-year old was born in 2000 the staff member had made the mistake of only checking the year not the date on her licence.
"We're just totally lost as to why he's made this mistake,'' Mr Smith said.
Mr Smith said he believed the employee's poor maths skills had also been to blame for the till being short at the end of the night on occasion.
Judge Kevin Kelly said the staff member had made a "significant mistake'' and selling alcohol to a minor was a serious matter which could not go unpunished.
Judge Kelly said the changes Mr Kelly had made to the bar's processes since the incident, which included sticking a date card on the till, were "laudable''.
The bar's licence will be suspended for 48 hours from May 9 and Mr Smith's suspension will start on the same day.