There is no response to "Céad míle fáilte", it's only used in writing (on signs etc.) or when giving speechs or welcoming large groups of people who are not expected to reply.
kaed meela fault-yih
"kaid meel-aa fall-cha"This is the way I'd spell it phonetically. 'Kayd meela fawlt-cha'. Try to say it quickly."kaid meel-aa fall-cha""kaid meel-aa fall-cha"