She bragged she was staying at New York's famous Waldorf Astoria hotel and bought jewellery from Tiffany & Co and airline tickets.
Prosecutor Pamela Brady said suspicions arose after a staff member at the firm she worked at noticed a "discrepancy" with an invoice and asked Mooney to locate the original.
"A number of weeks went by and the accused did not produce the invoice. She said that she was too busy," Brady told The Sun.
A probe in to the unlocated money, uncovered transactions dating back to April 2014, with multiple payments spent on payments to loan companies, leisure activities, restaurant bills, flight costs and significant spending in New York City.
Brady said the managing director of the firm called a meeting with the accused and the company solicitor who later admitting to stealing the money.
"She stated that she had financial problems," Brady said.
Defendant Ellen Macdonald said Mooney had taken on "family debt" after the sudden death of her father.