The keyring is an item that makes use of keys much easier in two ways: The keys no longer take up any space in the inventory, and by pressing the 'k' hotkey, all keys in the keyring are tried at once, so there's no need to look for the right key any more.
In the patched version of Ultima VIII, a keyring can be found right in the beginning of the game, from a barrel in the docks, and for the unpatched one it's on the lower left side tower guard after crossing the wooden gate from the castle at Central Tenebrae. There are also other keyrings to be found elsewhere in the game.
In Exult, keyring-like functionality can be got in two ways for Ultima VII: The Black Gate, where there is normally no option to use this convenient item. First, by default, the same 'k' hotkey can be used to try every key in the party's inventory to a lock; the keys still take up the same space, but this saves the player from the trouble of trying the keys individually. The other way is to play the Exult Keyring mod, which adds a keyring similar to the one in Serpent Isle to the game.