Cader Idris CAH-dir EE-drees
Welsh: The Seat of Idris

Named "Seat of Idris" due to the odd shape of the mountain (it looks rather like a couch), Cader Idris is one of the mountains of the Snowdonia range in Northern Wales, the ancient Gwynedd. It is about 2,927ft in height,standing at the southern gate of Snowdonia, overlooking Dolgellau. The three peaks are Pen y Gadair (Head of the Chair), Cyfrwy (the Saddle) and Tyrau Mawr (the Big Heap). The at the foot of the mountain is also the supposedly bottomless Llyn Cau (Lake Cau).

Legend says that it was the home of the giant Idris; three large stones lay at the bottom of the mountain, and it is said that Idris, in a fit of rage, kicked them down the mountain.

Other legends say that anyone who spent the night in the hollow of the "seat" would either die, go mad, or be a poet. The facts behind such a legend may be that, as Druidism had the Isle of Anglesey as its home base (according to the Romans, anyway), and Gwynedd (at least in the fifth and sixth centuries) ruled Anglesey (eg. Maelgwn Gwynedd, Peredur), the site may have been sacred to the Druid religion, a type of Sinai where students went to learn Bardism before moving on to the final stage of Druidism¹.

Others have said that the mountain is Arthur's Seat (one of many mountains called this).

This is also the mountain which Susan Cooper describes in The Grey King, where there the Seven Sleepers must be awoken by Will and Bran. (see The Dark is Rising)

Felicia Hemans (a Victorian poet I really don't like) wrote of the mountain in the poem "The Rock of Cader Idris":

I LAY on that rock where the storms have their dwelling, 
  The birthplace of phantoms, the home of the oloud; 
Around it for ever deep music is swelling, 
  The voice of the mountain-wind, solemn and loud. 
'Twas a midnight of shadows all fitfully streaming,
  Of wild waves and breezes, that mingled their moan;
Of dim shrouded stars, as from gulfs faintly gleaming;
  And I met the dread gloom of its grandeur alone. 

I lay there in silence­a spirit came o'er me;
  Man's tongue hath no language to speak what I saw:
Things glorious, unearthly, pass'd floating before me,
  And my heart almost fainted with rapture and awe.
I view'd the dread beings around us that hover,
  Though veil'd by the mists of mortality's breath;
And I call'd upon darkness the vision to cover,
  For a strife was within me of madness and death. 

I saw them­the powers of the wind and the ocean,
  The rush of whose pinion bears onward the storms;
Like the sweep of the white-rolling wave was their motion,
  I felt their dim presence,­but knew not their forms ! 
I saw them­the mighty of ages departed­
  The dead were around me that night on the hill: 
From their eyes, as they pass'd, a cold radiance they darted,­
  There was light on my soul, but my heart's blood was chill. 

I saw what man looks on, and dies­but my spirit
  Was strong, and triumphantly lived through that hour;
And, as from the grave, I awoke to inherit
  A flame all immortal, a voice, and a power !
Day burst on that rock with the purple cloud crested,
  And high Cader Idris rejoiced in the sun;­
But O ! what new glory all nature invested,
  When the sense which gives soul to her beauty was won ! 

NOTES:

1. The idea that Cader Idris was a spiritual center is purely my own speculation, and cannot be taken as fact, only theory.