From this discussion, it is reasonably clear that the Rose Cottage of relevance to me is the one on the map above, on the road parallel to the river heading up the hill to the castle and church, the road now called Castle Hill.
However, I did a google search for 'Rose Cottage St Fagans' and discovered that there is a grade II listed building in St Fagans called "Rose Cottage"! That one is north of the castle and church, on Crofft-y-Genau Road! Sadly, nobody seems to have written a history of that listed cottage.
So, for later searchers looking for Rose Cottage in St Fagans, be careful about which Rose Cottage you are after.