In reply to pince22:
I'm sure you've looked at a map and seen this - but the earlier poster saying you can pop over a col behind the hostel and wind up just E of Tryfan is a little over simplified! You can bear north then west and come out E of Tryfan but the col is at 750m and some is cross-country.
There is a nice walk from the hostel, NNE to LLyn Cwmffynnon, skirt the E shore and along the Bwlych Dwyglydion to the summit ridge between Glyder fawr and Glyder Fach. Then down y Gribin and over towards Milestone Buttress. It's probably 3 miles and a fair slog up.
If you're set on the N ridge of Tryfan then a lift, a bike or a hitching partner would be best.
Could you stay at Ogwen YHA instead?
Post edited at 20:54