* Rua (roo-ah): This means "red," and would be appropriate if referring to something rusty due to its reddish-brown color. This is probably the closest single-word equivalent for a rusty object.
* Meirgeach (mer-gyach): This word means "rusty" in the sense of being affected by rust. It's a more direct translation if you're talking about the state of being rusty.
* Caite (kah-teh): This means "worn" or "used up," and could work if you're talking about skills being rusty, like "My Irish is caite." (My Irish is worn/rusty).
Therefore, the best translation depends on *what* is rusty. For a rusty object, rua or meirgeach are good choices. For rusty skills, caite is more suitable.