I am not that good at mythology but,
Aos Si or Aes Sidhe in old form, is a mythological race. They are royal descendant. When they first came to Ireland they fought against a race that already occupies Ireland. That race was demonic nature and they banish them and occupy the Ireland themselves then humans came and fought against Aes Sidhe, humans won and forced Aes Sidhe to hide. When they forced to hiding they became more malevolent. Then, when Christianity first arrived on Ireland, they wanted to erase their history.
They are known for their otherworldly haunting beauty, prowess and skill with almost anything, iirc they are talking to animals or can communicate with them, also either immortal or immortal like lifespan. They also can shapeshift iirc.
A few key point of them, its been told transitional state of the humans are most interesting to them. Pregnant women, newborn baby, puberty etc. They are also mainly rural folk they dwell in forests, hills etc.
I think many fits to Fiona and what she told about her family, so I always assumed this.
Take these with grain of salt though, it's been long I've read about them. First I saw Fiona and read her story, I immediately thought this but I may have misremember some part or mix with some other mythological elements.
Also, iirc banshees are part of Aes Sidhe so as Leprechaun.