Bron Yr Aur is not Open C tuning, Open C would be CGCGCE. There you have the root note, the 3rd and the perfect 5th.
Bron Yr Aur is played in Open C6 which is CACGCE, here we have the root note, the 3rd, the perfect 5th and a major 6th.
The song is in 4/4 time, so if you are having trouble finger picking, take it slow. Play the bass note with your thumb.
E|------------------------|
C|----------0-------------|
G|-------0-----------0----|
C|----0-----------0-------|
A|-3----------------------|
C|-------------0----------|
p i m a p i m etc
p - thumb
i - index
m - middle
a - ring
play through it slow and keep practising so that you build up stamina.
Achilles