Semi-litearlly, this is "one thousand thank-yous", or even more litearally, "that one thousand good to you".
"May 1000 thousand good be at you"
why not Go raibh milliún maith agat? :D or maybe just céad míle
You just add míle haha. Go raibh míle míle maith agat
The question said nothing about a 1000