About this phrase

Go is a particle which is used to form adverbs from adjectives.

Maith is an adjective that means good.

Thus, go maith is an adverb that means well.

Notes

In English, the words good and well have become somewhat interchangable, albeit to varying degrees across the dialects. Consequently, the expression of the concepts 'to do / be well' and 'to do / be good' have become more context-dependent:

"How are Count Dracula, Nurse Ratched and Professor Moriarty?"

"They're all good!"

In Irish, the particle go makes a clear distinction between the two:

• Fás maith (good growth).
• Ag fás go maith (growing well).
• Obair mhaith (good work).
• Ag obair go maith (working well).