that it's an act짓 means an act, deed, or thing someone has done. 이라고 marks a quoted statement; 짓이라고 often appears after an adjective, such as 위험한 짓이라고, meaning “that it's a dangerous thing.”
KO: 그건 정말 무책임한 짓이라고 생각해요.
EN: I think that's a really irresponsible thing to do.