Using ”ag” for at, -ing, to have

Ba cheart go mbeadh a fhios agam ar rudaí bunúsacha tar éis scoile, ach ní raibh. Is dócha, nach raibh mé ag éisteacht.

For example I didn't quite understand many of the things that "ag" can mean.

• Ag means "at" and can be used like at in English
• It is also used like "-ing" at the end of a word in English
• It can mean to have something