She's too irresponsible to be relied on.

Literal

She [topic-は] too-irresponsible [because-ので], rely can-not [assertion-よ].

無責任すぎるので、あてに出来ないよ — すぎる marks excess ('too irresponsible'). あてにする ('to rely on, count on') in negative potential. The よ adds casual assertion.