Sean Bernard Downey pleaded guilty to the attempted robbery of Stott and Willgrass Jewellers when he appeared via video link from prison in the Royal Court on Friday.
He denied a charge of having an offensive weapon – a broken bottle – on the same day, 21 July.
Although accepting that he had had a bottle with him, he said he did not intend to cause any injury.
The pleas were accepted by the Crown and a sentencing date before the Superior Number of the Royal Court – which only sits for the most serious cases – was fixed for 8 December.
Downey was remanded in custody until that date.