Not so bad
Pretty good. Irish understatement that upgrades a thing without admitting enthusiasm.
How was the match?" "Ah not so bad now. We stuffed them.

Shall I translate it into posh English for you, horse?
Ah, welcome back. Or welcome. Either way, welcome. Here it is in posh English.
How was the match? Rather satisfactory, as it happens. We defeated them comprehensively.
Questions
What does not so bad mean in Ireland?
Pretty good. Irish understatement that upgrades a thing without admitting enthusiasm.
How do you use not so bad in a sentence?
“How was the match?" "Ah not so bad now. We stuffed them.”
Updated
On the list: Irish slang phrases