From ESOUI Wiki
string displayName GetDisplayName()
The function returns the account name as version to display, basically the account name with the prefixed @.
The account name of the player for display purposes as string, i.e. "@AccountName"
local myname = GetDisplayName() MyLabelControl:SetText("Your account name: " .. myname)
With the 1.2.3 patch this function has been broken and returned a empty string on the Live Server Clients (this was not intended). This caused havok with the saved variable system (among others). The issues was fixed with 1.3.3 and never appeared on the PTS servers.
There are several alternative but no 100% reliably way to get the DisplayName. Several have been used in a temporary fix for GetDisplayName: http://www.esoui.com/downloads/info614-DisplayNameFix.html