Hey, I gotta tell ya, don't listen to 80% of the crud you here about the Sidhe. The word itself causes problems. One can't always tell if we are talking about some kinda faerie people, the hills faerie people allegedly lived in, regular people who lived near burial mounds, dead people in burial mounds or simply...hill people.
All this stuff about wearing white, seelie and unseelie, faerie food (no offence) etc... is simply romanticism.
You're best bet is to read the stories of the Inavsion Cycle of Ireland. Oh....and read the mabinogi (welsh).
In the Invasion cycles, they talk of the tribe called the "Tuatha de Dannan" These connect to the sidhe in one of two ways (that i have found). In one, they BECAME the Daoine (or Aes) Sidhe by either going through specific hills (Like New Grange, which is not a natural hill) to live in another plane, to go into the land of the dead (because they died after being defeated by the Milesians), or because they simply went to live in the hills. (see above difficulties with the word "sidhe")
The second way they connect is that they encountered the Sidhe after being driven from Ireland. Again, it is either because they went into another plane, or died and went to the land of the dead (hence..ancestor worship prevalent with Ireland's tribes).
The Mabinogi has Arawn..and interesting figure with the ability to alter his appearance and the appearance of others. he rules the land of Annwn, which seems to be on the "other" side. It seems that at some point Arawn became a Lord of Death and Annwn the land of the dead...not sure why though, nothing in the Mabinogi hints at those notions.
When reading the above two (and you could include the Fenian cycle and every concerning Chu Chullain (spelling varies) ) look for animals with white fur and red ears, dapples, spots, tri-colored hair, and I'm sure..gold brocades of silk....these are all devices that seem to mark things otherworldly.
Finn's son Oisin in particular seem to encounter these otherworld people a bit.
Now...I myself have observed a specific energy or whatever that associates with the idea of 'Sidhe." It differs from the dannan, humans, and the Alfar. Interestingly enough, I pick it up again within the concept of the elusive "picts" of britain. Because of this, and because of ol Arawn, I almost link "sidhe" more with Britain for some reason. I'm also sure I will be spat at for this..hahaha