He went into "bandit country" with an eye to meeting up with enemies to talk of possibilities of peace. Couldn't exactly do that up in Dublin, or even in Cork City which had been cleared of enemies at that stage.
He went with an armoured car and a detachment of troops and as it happened had the enemy at the engagement pretty well outgunned - especially had the machine gun not "jammed". Travelling in an open touring car probably wasn't the wisest move at the time but he wanted/needed to meet with and talk with his former comrades now enemies, or in O'Donnoghue's case "neutral" but pro-IRA.
Relaxing his guard in his home village and local was perhaps understandable. Buying rounds for the crowd was the kind of thing he did. Imbibing, and then over-imbibing himself, was reckless certainly. But that's the nature of taking a pint, it can lower your inhibitions about taking another one, and it certainly didn't make him a stupid person.