I believe it is Dé Danann, where the Dé means "god." In some manuscripts they are referred to as simply the Tuatha Dé. But here's the thing: I haven't seen any of the actual manuscripts or even facsimiles, only transcriptions, so I can't be sure how it was written in the middle ages.

Interestingly, the band changed the spelling of their own name from Danann to Dannan, during their career. There is one album where they are making the switch, and they use a sort of double n character to obscure the spelling! I can't remember the album title, but it's one of the ones with Mary Black.