Sweet fuck all really. Maybe just a fine. Laws on marijuana are fairly lax in the uk and ireland. We have it good my friend but not for long.
It works on a 3 offences system, 1st offence - slap on wrist. 2nd offence - stern warning and maybe fine, 3rd offence - can mean jail time but usually on probation. Very uncommon to actually go down unless your involved in organised crime.