Why is thunder and lightning rare, even though it rains frequently?
Rain in a climate such as Ireland is most often stratiform. That is, it is created when moist air is lifted by a front, or by divergence in the upper troposphere. Thunderstorms involve buoyant air, which rises on its own due to its being warmer than its surroundings. Typically solar radiation is involved in heating the surface such that the air near it warms and becomes buoyant. That is more common in lower latitudes and locations that are typically sunny.