In English prisons in the 17th and 18th centuries, the prisoners used to pay money to the jailers in return for better treatment. This money was called garnish. The word garnish was already used in mainstream English by this time for a little extra something added, a ‘bit on the side’, so the use of garnish as a sweetener for a jailer is easy to understand. There is no mystery here, nothing to be explained.
This didn’t stop Daniel Cassidy, the Great Fraud, from trying to explain it through Irish in his crazy book How The Irish Invented Slang. As usual, his attempt was totally incompetent. He claimed that garnish (in the sense of sweetener) came from the Irish garanna ar ais, favours back. In theory, this could be an Irish phrase. The word garanna does mean favours and ar ais does mean back.
The problem is that there is no evidence that anyone has ever actually said garanna ar ais in an Irish conversation. Try looking up the phrase on Google. I found one reference to Cassidy. Nothing else.
Then try putting in some of the real phrases used to describe paying back or making restitution: cúiteamh a dhéanamh le, an comhar a dhíol le. Or what about the word for a tip, síneadh láimhe? These produce a handful of results from different sources because they are real Irish, while Cassidy’s phrase is just made-up nonsense.