How do you access a key you need once it's on the keyring?

As I recall you drop the keys from your inventory onto the keyring and then when you need a key you just pick up the entire keyring to interact with the desired object and it just uses the right key. Its been a while since I have played but this is how I remember it working.

Yeah, just use the keyring on the lock and you will unlock it, provided you have the correct key on the keyring.
For keys that you picked up before you found the keyring, you have to manually put them on the keyring by dragging them onto it in your inventory. Once you have the keyring, if you find a new key somewhere, and you pick it up using the RIGHT mouse button (the one that automatically puts the item in your inventory) it will automatically be added to the keyring.
When I first found the keyring I got confused because I tried to use it to open a lock and it didn't work, but it turned out that I actually hadn't found the appropriate key for that lock yet. Using the keyring on locks definitely does work when you have the right key.

Bingo! Thanks, people...