You want to alight from the train at Wrexham. Judging by train times I think its equally easy to get there from Manchester, Liverpool or Birmingham.
When you get to Wrexham you want to take the X94 bus which runs all the way to Barmouth.See :-
www.arrivabus.co.uk/serviceInformation.aspx?id=2501
You need to get off at the small village of Llandrillo which is about 5 miles after Corwen.
Theres a very posh place to stay in Llandrillo:
It's rumored Prince Charles has been there a few times to eat. Apart from that I dont know anywhere else, but im not saying there isnt a B&B nearby.
There's this place in the next village:
http://www.brynmelynfarm.co.uk/home.html
But if you dont have a car you are going to be very restricted being here.
You are probably best staying in Corwen, a pleasant small Welsh town where there are several places to stay. You can then get a local bus or a taxi down to Llandrillo eithout much difficulty and with limited expense