There isn't one. Ireland also doesn't have a national ID program, so effectively for most people their passport or driver's license are the only forms of government ID they _might_ have. But even then it's not mandatory to have either.
"EU id" was a shortcut for "id documents issued by EU countries", they're valid EU-wide. It's an ID card in most EU countries and that's what would be used for the purposes of this article, except for Irish citizens I guess but they are probably a relatively small percentage of EU visitors of porn sites.
Now I just noticed that although the subject is "most of the EU", this is an Irish site, so maybe they just didn't really take this into account.
