You shouldn't go out with someone who is irresponsible.
It's not a good idea to date someone who is undependable.
この場合、次のような言い方ができますよ。
ーYou shouldn't go out with someone who is irresponsible.
「いい加減な人と付き合わない方がいいよ」
someone who is irresponsible で「いい加減な・無責任な人」
ーIt's not a good idea to date someone who is undependable.
「いい加減な人と付き合わない方がいいよ」
someone who is undependable で「いい加減な・頼りにならない人」
ご参考まで！
カナダ 
