Even combined forms in plural (eadrainn, eadraibh, eatarthu) aren't used with "agus" today:
"between you (pl.) and them" = idir sibh agus iad (instead of eadraibh agus iad.)
"between these and those" = idir iad seo agus iad siúd (instead of eatarthu seo agus iad siúd)

(except some petrified phrases like: Dia eadrainn agus gach olc)

That's probably the reason why eadram, eadrat fall out of use.
(Perhaps in some forms of mental disease forms like "d'éirigh eadramsa" might still be appropriate)