Just that. They are a sort of additive brake. The buffer stop is merely clamped to the rail such that it will slide, if hit hard enough, but there is quite a bit of friction involved, due to the clamp blocks. If the clamps on the stop itself don't have enough friction to halt the progress, then, the whole assembly will start to slide on the rails. It will then successively 'collect' each pair of additional brakes, thus adding to the friction available in a series of steps, and thus providing a very high braking force when it becomes needed, but not applying such a high force for a smaller impact.
To some extent, though, it does rely on those fish-plate bolts surviving the additional force pulling the rails apart, too. That may be why there's a bit of additional ballast - to support each sleeper sideways...
Note:- Always check the time of arrival of the next train from Nenagh before using the gents' at Ballybrophy!!
Those new stops at Heuston are bolted through the rails - you're on your own if you hit them. I suppose that, where they are, there is very little likelihood of them being hit at any real speed - or with passengers aboard..?