iawn mate, the route i got is not directly off marin the route is come out of the gwydr forest at ty thll then down to maesneweddion (I think) then upto Llyn Elsie come down Elsie from far end of lake the forest track will take you down to betws y coed from there upto penmachno via side roads. that's all i know. hope it helps.
Another way is the Sarn Helen trail (Old Roman Road). Cross over A5 by Ty Hyll and take right over river on B road up towards Pont Cyfyng. Left by farm 3km up and climb up track for approx another 3km into forestry. Decsent takes you down into Dolwyddelan. Cross the A470 and climb again on Sarn Helen (more or less straigh on after bridge). Remember to hang a left half way up the climb and head towards left edge of the Forestry over the top (more like straight on, but other track goes off to the right up towards y Ro Wen). Boggy decsent brings you out on Loop 2 of Penmachno trail. It should be shown as a trail most OS maps (either green or red dots).