Synonyms for Irresponsible (same or very similar meaning)
WordNet sense 1 (lacking a sense of responsibility):
unreliable
WordNet sense 2 (free from control or responsibility):
unaccountable
WordNet sense 3 (lacking a sense of restraint or responsibility):
loose
WordNet sense 4 (not fit to assume responsibility):
feckless
WordNet sense 5 (cheerfully irresponsible):
carefree
From the ODE community, based on WordNet
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