It’s both, but it depends which country you are in, or rather which Gaelic you are speaking. My Irish dictionary has it as Ceili.

It’s Ceili in Ireland (or with Irish people) or a Ceilidh with whisky if you are in Scotland (or with Scottish people)!

To be frank either spelling is good! What it means is a great time with lots of dancing and fun  Anyway here are acouple of images from last nights Ceili or Ceilidh