Off the coast of the continent is the Isles of the Mysts. It is a moderate sized isle with stranger properties.
First off, it is not on any current map of the Continent. It can not be reached by simply sailing in a direction. It can be reached only when the moon is right and the ebb and flow of time is favorable. In fact, if you go to the right place in the world at the right time, you can walk there.
The Isle has similar geography and weather to Ireland. The people there are of celtic origin and culture, circa around the time BC. They call their island, the Isle. The only real difference is the presence of more fog.... Well, and the other places.
The other places. If you walk the right paths at the right time, you can get to the other places. Geographically, they are much like Ireland. Weatherwise, much the same, except for the fog. But each other place is a bit different.
The closest "other places" are the easiest ones to get to and have the most contact with the Isle. Pathfinding or Mystwalking is a feat or knack to find ways to other places. You can do it without knack, it is just more difficult. (if you take it multiple times, you get better odds)
The summer country, is a land of Shidhe. These are not the Tolkein bastardization of Shidhe, but real Fey. They are taller, stronger, finer of feature, sharper of wit, and wiser in the way of magik and war. In short, they are everything humans want to be and a bit more.
North Country, is another Fey land, but one of ice and cold. Snow Wizards and Ice Princesses, Yeti and Polar Bears, populate this land.
The Swordsland: These land is populated by a people from far and away. They are masters of the sword. (Imagine a world of idealized Samurai). This may be an optional one, it is not celtic, but it does fit the theory...
Avalon: Here the high king and his knights maintain a beautiful and orderly society.
The Wild Lands: Filled with stange beasts, and stranger subhumans.
Note: To reach an otherland from an otherland, you have to cross into the Isle and find the way to the other otherland.
The Isle of the Mist is a place of dreams, where ideal times and places are kept, to keep the dream of their time and place alive. It is a magikal place filled with ideal places.
Now of course it has to menaced by some great Evil, trying to corrupt and take the magik of the Isle.