Until 1980, the most notable feature of Ireland's labor relations system was:

A) The prevalence of strikes
B) Peak-level bargaining
C) Enterprise unionism
D) A high union density rate

Until 1980, the most notable feature of Ireland's labor relations system was the prevalence of strikes, which were more common in Ireland compared to many other countries during that time period. This was a significant aspect of the labor landscape, reflecting the contentious nature of labor relations and the willingness of workers to engage in strikes to press for their demands.