Thanks for replying everyone!
In Ireland, our roads are disgraceful. In the very rare case where there is a hard shoulder, it's usually of very poor quality and full of holes (which destroy my bike, but that's another issue). Also, drivers generally have no consideration for the cyclist, and give us very little room. So, listening out for the traffic is very important. Despite this i do like to listen to music while i cycle, it helps pass the time, but i'm serously considering stopping this practice.