Irresponsible, a.
Etymology: [Pref. ir- not + responsible: cf. F. irresponsable.]
1. Nor responsible; not liable or able to answer fro consequences; innocent.
2. Not to be trusted; unreliable.
Shades of meaning
Fernald has no entry of its own for irresponsible; he lists it under absolute.
Where he writes about it:
Irresponsible power is not necessarily bad, but eminently dangerous; an executor or trustee should not be irresponsible; an irresponsible ruler is likely to be tyrannical. — from absolute
A perfect ruler might be irresponsible and not tyrannical. — from absolute