Basically it's illegal and it's not decriminalized. If a guard (cop) finds you with marijuana there are a number of things he might do. His attitude towards weed will play a big part along with your own attitude. Generally speaking if it's under half an ounce they will confiscate (some may pursue the matter further) and dispose of it. If you are smart and cheeky it will probably work out better to keep your mouth closed and be honest.
If there are aggravating factors such as smoking near a school or playground or your are found to be selling it i.e you have text message from people asking for specific amounts or this isn't a first offense. You will have to pay a fine and possibly go to court.
Very unlikely you would ever go to gaol for smoking weed unless you we're taking the piss. Gaol is generally reserved for serious offenses such as violent orientated ones.
If you grow very small amounts and smoke a bit in your own home. Unlikely you would every be caught and if you were and it's obvious it's for personal use a blind eye is generally turned.
The way I see it, things are far better than certain countries and certain US states but not near as good as places like Calif and B.C and Amsterdam.
While the laws are strict, I would honestly say Ireland probably has the most relaxed police in the world. I don't know any other countries were the cops put up with the same crap Irish ones do. When I got to the States, Britain or France. I genuinely fear the police and I don't even joke with them. Friendly as many of the police are in the States they have little patience for messing.
(Just to note there are plenty of isolated cases of abuse by the guards but I'm speaking in general terms.)
