The phrase appears in both Irish and Scottish Gaelic. In both, it means "A Hundred Thousand Welcomes". In Irish, it's spelled Céad Míle Fáilte. In Gaelic (Scottish), Ceud Mìle Fàilte. That's...
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.