Was this irresponsible?
Of course it's irresponsible. The purpose of giving suitable notice is to allow your employer sufficient time to plan for your exit, and smooth the transition to your successor, if that is possible.
How long is an appropriate notice for a pastor to give his resignation?
Our previous senior pastor gave about two months. That seemed short to me, but then again, he was
there for 15 years! Also, the elders were notified before he went public.
What is appropriate is sufficient time for the church leaders that are left to plan for his absence - lining up guest speakers, organinzing a pulpit search committee, and so forth.