Hump yards are used to assemble freight trains with wagons/cars from different sources. A shunter will push the wagon over the hump, signalman will select the appropriate track for it, and a pneumatic retarder fitted between the rails will slow it down just enough so that it rolls up to the rest of the wagons and buffers-up (or couples if automatic couplers are used). The retarder was operated by a staff member in a tower overlooking the yard, although I think modern ones are automated.
There were none in Ireland but the UK had some, although I'm not sure if any are still operational there. There's quite a few in the US... here's a video of one in operation: