The IMRA organise a number of events as well in Cork and Munster and they are usually on trails on various hills and mountains. A list of all off-road races can be found in the Munster Race Calendar in Green Italics.
In a recent issue of Runners World, there was an article about Trail Running in Ireland. Written by Lisa Jhung, it gives a unique view of the sport as seen from an American point of view. The piece can be seen HERE.