Shamus is a leprechaun whom the player is required to talk to during the Lost City quest. He lives in a tree in Lumbridge Swamp, which the player must chop in order to get him to appear. When spoken to, he tells the player where to find Zanaris and how to enter.
The tree he lives in is located due west from the adventurers that start the player on the Lost City quest. The player can find it easily, because its left-click option is "Chop Tree" instead of "Chop down Tree".