Did the Garda say he was going to bring you before the courts, I am no expert on the matter, but I thought if it was for a small amount and clearly for personal use they only give a warning, I have no time for drugs or people that take them but it seem a bit harsh to drag you through the courts system, in any case I would expect a donation to a charity of the judges choice, I wouldn't bother with a brief, when your case is called just admit your mistake and say sorry it was a one off, and they will know your record before you set foot in the place, if what you say in your post is all the facts you will be fine