Otherworld
While Annwn/Annwfn--the Otherworld as ruled by Arawn--isn't specifically said to be inside a hill, it is likely that that is the case in the earliest version of the story. In alternate tales, when Annwfn is ruled by Gwyn ap Nudd, entrance is gained through Glastonbury Tor (see "St. Collen and Gwyn ap Nudd").
Now, when a human finds his way into these sidhe, it is usually on one of the "Fire Festivals": Samhain, Imbolc, Beltane, and Lughnassadh. On these days--but especially Samhain and Beltane--the doors on the hills would open and the faeries would walk about the realm of men. There are numerous stories, both literary (such as those dealing with Fionn mac Cumhill) and folklore (too numerous to mention) wherein the hero witnesses the faeries leaving and entering the hollow hills on these days, participating on combat on these days, and so on. This method seems to be discouraged by the faeries--this is their time, and the human who stumbles upon them is often punished in some form.
The other method of finding a way into the sidhe was to be lost: caught up in a magical fog--such as when Conn C�ad Cathach and his men were caught, and found themselves at the house of Lugh Lamhfada, or lost in the woods, such as when Pwyll ran into Arawn while hunting. This is usually a case of being selected by the god for a special reason--kingship, or switching roles for a duel. In these instances, the hero is (eventually) rewarded for his deeds.
What is most important about the s�dhe is that while it is supernatural, ruled by gods, it is very much like our world--there are wars, jealousy, betrayals. It is as full as strife as our world, and often this strife bleeds over into our world.
The term Hy Breasil (and its various spellings) refers to the Blessed Isles of legend. Until the modern era (well into the age of exploration), maps would often have an island of "Brasil" or "Breasil" out in the Atlantic. The idea of islands out to the west is, of course, half based in fact and half based in myth: the Hesperides and Hy Breasil could just as easily be the Azores and the Faroe Islands. So it is no surprise that the Celts had islands in their Otherworld; it is important to notice, though, that they differ with the sidhe in both their governance and their reachability.
According to Irish (and occasionally in Welsh) mythology, the islands lie "beyond the ninth wave." Now, the reason for this is likely bound up with their fondness for the number three--nine, of course, is 3 x 3--a square of three, probably representing perfection (this is just speculation). The Otherworld islands are lands of peace and eternal life, unlike the sidhe:
Without grief, without sorrow, without death,
Without any sickness, without debility,
That is the sign of Emain--
Uncommon is an equal marvel.
--The Voyage of Bran
In Irish myth, these islands are ruled by Manannan mac L�r. Now, Manannan, while a god, is not really one of the Tuatha D� Danann. He has no pedigree with them, but instead is descended of Ler, "the sea" about whom little is known, though he is the later basis of Shakespeare's King Lear. And so, as the isles are not ruled by the TDD, they are peaceful, not centered on the inter-tribal warfare of the sidhe. They are also extremely difficult to access. One only reaches these islands through the invitation of Manannan or his daughters. This process is usually termed an echtrae.
In The Voyage of Bran and in other Irish texts, we are give several names for the Otherworld Islands:
Emhain Abhlach: Plain of Apples
Mag Findargat
Mag Argatn�l
Mag R�in
Mag Mon
Mag Mell
Aircthech
Ciuin
Emne
Imchiuin
Ildathach
Inis Subai: Isle of Joy
T�r na mBean: the Land of Women
T�r fo Thonn: the Land Beneath the Wave
T�r Tairnigir: the Land of Promise
T�r na nOg: the Land of Youth
Hy Breasil / Hy-Brazil / Hi Brasil: Most Best Place (?)
The Voyage of M�el D�in (though a Christian immram) lists some forty islands, while The Voyage of Saint Brendan lists still more islands; both texts are similiar to the early, pagan echtrae in their descriptions of these islands.
For a human to come to these islands was, except in certain circumstances, to be there for eternity. Connla and Ois�n were chosen by lovestricken daughters of Manannan. While Connla was never seen again, Ois�n foolishly decided to return to Ireland to see his family again; he found that hundreds of years had passed in the interim. Moreover, once he dismounted from his enchanted white horse and set foot on the soil, those hundreds of years caught up with Ois�n and he died. Similarly, Bran, after visiting the various islands mentioned above, returned to Ireland only to find that he had been away for hundreds of years; he sailed back out to sea, never seen again.
More pleasant is the story of Cormac mac Airt, who was brought to the islands by Manannan so as to give him the cup of truthfulness and other magical items. Cormac was able to return home, as he had been invited there for a short time, not intended as a husband for one of the faeries.
In Welsh myth, though the islands are less prominent, they do exist, more or less ruled by the enchanted head of Bendigedfran ap Llyr, brother of Manawyddan (the Welsh Manannan). Here there is no aging, no death, great feasts, and the ability to forget all pain. According to Branwen uerch Llyr, he and his companions stayed on Gwales (an unidentified island) for eighty years in this state, before one of them broke the enchantment by opening a forbidden door.
In some medieval Welsh traditions, Myrddin gathered the Thirteen Treasures of Britain and housed them on his "Isle of Glass."