I was impressed by the speeds at first - but it seemed all the cars were braking in front of him -why would they do that??
1) they saw the cyclist and slowed to let him past - a possibility but probably not the case here. It has happened to me on more open less complicated foreign roads where cyclists are commonplace and people in cars aren't in a hurry
2) they are cautious drivers, possibly older aged - could be so for country lanes in Ireland
3) they know the road, they know the steep gradients going round the corners, the blindness of the bends, how narrow the lanes are for them and that to get round safely allowing space for oncoming traffic, they need to be at a low speed to manoeuvre and on the brakes. They have no margin for error while in the narrow lane going round a tight bend on a steep gradient. I think this is what's going on and also explains how the cyclist can get his acceleration so quickly.