Not sure what you mean by 'come across' it Rich. It's the default situation, it exists unstated! It's everywhere! You'll see signs where it no longer applies and then, further up the road, where it's been re-instated. Or it can be overridden at marked junctions (give way or stop signs or markings) but at every single unmarked junction you'll have ridden through, it applies.
Sometimes (perhaps even mostly), you'll see unmarked junctions signed in advance (a simple black cross sign, lines of equal weight) to make you aware of the junction. It also tells you that priorite a droite will apply.
In practice (and this is perhaps your point), it's not an issue since the majority of such junctions that the cycle tourist will come across are invariably clear, such is the lack of traffic. But all the more reason, perhaps, to be aware of the law. You may think you're on the 'major' of 2 intersecting roads and therefore assume priority, only for the car on your right to sail right through the junction! It pays to be aware.
Equally important, particularly in rural areas, is to be aware that many of the older drivers (or those that have never come across anything other than priorite a driote) will sail through marked junctions too, regardless of giveway or stop signs!
Believe me, priorite a driote exists! And you'll soon become aware of it (hopefully not painfully) when you've made an incorrect assumption!
