did they ever say what became of her? I know they never did with Frank.

Irene's last episode was Gloria's baby shower in late 1975. Garrett I believe left the show since she was given very little to do. Then of course she went on to Laverne and Shirley. No explanation was given for Irene no longer being seen. For all we know she might've been living next door to Archie but no longer on screen post 1975.