When I was 17 I worked in a nightclub during the summer. I'm sure it was against some regulations or other but it was really just casual work that gave me pocket money.
I suggest looking for casual work for event companies (like bouncy castles), garden centres, corner shops, hotels and pubs (glass collecting - might get off with it if they say they're 17), or even to get work with a painter or tradesman. The pay will be peanuts but it would be good experience for the future. Don't expect minimum wage or even prsi contributions to be paid by the employer.
Another option is to get a lawnmower and strimmer and some petrol and go around houses in the area offering to cut grass.