There is a sign posted in A Ponte, which illustrates the way to Monforte. There are various routes in these parts, and although most of them will eventually lead you to Castroncelos, only one of them is well marked.
Shortly outside of Barxo, after crossing over the bridge, a trail splits off to the right of the road. DO NOT take that trail, but rather head upwards along the wider road. It will take a few turns, and you may well get the sensation of going the wrong direction as it zigs and zags. Eventually, it will straighten out, and you will once again have the N-120 on your left at the bottom of the hill.
Ignore all signs for the ironical name GR-8.