The reason I ask is because I have travelled a few times recently on the M50 from Firhouse to Ballinteer and I seem to be just about the only one observing the temporary limit of 60 KPM.
Yes, there are speed cameras on the m50. My boyfriend got 2 penalty points for speeding on a stretch of road with a temporary speed limit, he wasn't paying attention and didn't notice the change in speed limit, he drives for a living so would never intentionally speed. People may not agree with a certain speed limit (or other rule of the road) but you still have to follow them, they're there for a reason.
I expect that if one was to end up in a District Court one might get done regardless. My view would be that in the District Court it's down to the whim of the Judge on the day. I suspect that one would need to be prepared to go to the High Court if hoping to use an 'it's not legally enforcible' defense.