While agreeing with Soy's excellent summary above, bear in mind that if you are considering driving your convertible on Irish roads, two major factors that are especially important are comfort and safety.
On smoother UK roads, the Mazda MX5 probably would be the car of choice though I would agree that it's not everybody's cup of tea! On Irish roads, the Renault Megane's suspension and better safety rating probably would make it the best option of the three.
However, if you are driving your convertible predominantly in Dublin, security would have to be your main consideration. I don't have any expertise on the seciurity features of any of these cars but would suspect the at MX5 is probably beloved of joyriders.