You need a traders licence if you wish to trade at a market i.e Moore Street this can be obtained through the local council. It is illegal to trade on private land unless you have permission, however you cannot be moved off this land unless the person who owns the land produces the paperwork to back this up, not even the Garda can move you untill they produce this paperwork. Finally in such councils as Fingal there is no such thing as a traders licence this means that it is up to the Garda to move you on once you are on Fingal land. They will only however do this if you are causing a traffic or pedestrian obstruction. If I were you and you chose to go that route I would get in touch with the local Garda station who would deal with that area and let them know who you are and what you are doing, this would mean that if a patrol car came around they would already be aware of you and once everything is above board I would say that you would be left alone.