dhw "Is God a killjoy? Is there any connection between non-belief in God and irresponsibility?"
I had a neighbor who was an engineer, a physicist and an atheist. He would rail at every opportunity against what he called "pleasure neurotic" religions, meaning Jewish, Christian and Islam. He was a heavy drinker, became an alcoholic, flunked out of AA twice and ruined his life before finally sticking with AA and turning it around.
His story is merely anecdotal, but it illustrates the question dwh poses. It also illustrates my much belabored contention of moral = wise. If he had drunk wisely instead of to excess, he could have retained his atheism and his sobriety both. He is still an atheist, by the way, although the twelve step requirement to confess to a power higher than himself gave him some trouble.
I think the answer to dhw's question is "yes, but it doesn't have to be." A belief in God is one source of morality, but good common sense is another. And the good common sense approach is a bit more flexible and adaptable than religious morality.
I firmly believe that religious morality is necessary only if one believes in heaven and wants to get there. For the rest of us, common sense will cover most situations.