"We are sad."
Translation:Tá brón orainn.
Note that "brón" is an noun, not an adjective.
You can use "brónach" as an attributive adjective ("scéala brónach" - "sad news"), but it's generally considered "béarlachas" to use it as a predicative adjective.
(Generally emotions/feelings are not used predicatively, but there are some exceptions - "tá mé tuirseach" is used, for example).