Keegan Davidson (26) assaulted the woman while she was at work on the shop floor of a busy Dublin department store and fled the scene.
Judge Bryan Smyth gave him a six-month suspended sentence and put him on a probation bond for a year.
Davidson, with an address at Focus Ireland, pleaded guilty to sexually assaulting the woman on a date in March 2018.
Judge Smyth said he had considered a prison sentence for Keegan but suspended the six months for a year after an updated probation report was submitted.
The accused acknowledged his signature on the bond before leaving court.
Previously, Detective Garda Maeve Hamilton said the victim, in her 20s, took up work on the evening in question in a booth at the store, selling designer clothes.
She saw a man approaching from her right-hand side, turned away to remove herself from his path and "the suspect slapped the injured party on the buttocks".
He then fled the store. The scene was covered by CCTV, which was downloaded and the accused was identified.
Davidson made full admissions and had written a letter of apology.
A victim impact statement was handed in on an earlier date.
Keegan had now "completely changed and turned his life around", his lawyer said.
He was "really apologetic and extends his sincerest apologies to the injured party", the lawyer added.
Davidson had been going through a "mental health situation" which had since resolved and was now in a "much more stable position", the court had heard.
The accused had previous convictions, including offences under the Money Laundering Act, theft, possession of weapons and drugs, but none for sexual assault.