"No somos responsables."
Translation:We are not responsible.
It would be more accurate to translate "Somos irresponsables." to "We are irresponsible.", I guess. Even though the meaning is close, "not responsible" and "irresponsible" are not always interchangeable. A synonym of responsible is guilty. "Not responsible" can mean innocent/non-guilty. Irresponsible and innocent, on the contrary, are not synonyms.
I think you guys should go with the most direct translation. The others are possibility entries but word for word it Mostly read "We are not responsible" Somos: implies the "we are" and the closest and most direct translation to responsible... is responsible.... (Forgive my errors)