The use of worn-out horseshoes as magically protective amulets -- especially hung above or next to doorways -- originated in Europe, where one can still find them nailed onto houses, barns, and stables from Italy through Germany and up into Britain and Scandinavia. Additionally, wall hangins made in the form of horseshoes are common. In the Middle-East, one finds the terra cotta blue-glazed horseshoe plaque. In Turkey small metal or blue glass horseshoes are blended with the protective all-seeing eye to form a unique apotropaic charm i call the horseshoe-and-eyes that is believed to ward off the evil eye.

There is good reason to suppose that the crescent form of the horseshoe links the symbol to pagan Moon goddesses of ancient Europe such as Artemis and Diana, and that the protection invoked is that of the goddess herself, or, more particularly, of her sacred vulva. As such, the horseshoe is related to other magically protective doorway-goddesses, such as the Irish sheela-na-gig, and to lunar protectresses such as the Blessed Virgin Mary, who is often shown standing on a crescent moon and placed within a vulval mandorla or vesica pisces.

In most of Europe, the Middle-East, and Spanish-colonial Latin America protective horseshoes are placed in a downward facing or vulval position, as shown here, but in some parts of Ireland and Britain people believe that the shoes must be turned upward or "the luck will run out." Americans of English and Irish descent prefer to display horseshoes upward; those of German, Austrian, Italian, Spanish, and Balkan descent generally hang them downward.

In regions where the horseshoe is placed facing upward, folks believe the horseshoe must point up "or the luck runs out." In places where it is hung facing downward they say exactly the opposite -- "it must point down so the luck can pour onto you." However, in its function as an amulet for magical protection, especially over the doorways of barns and stables, the horseshoe usually points downward and it is said that "no witch will pass under it." The "Good Luck" horseshoe image i use on my web pages came from a 1940s American printer's stock cut book, probably drawn and engraved by a German-American, hence the horseshoe points downward.

What does the difference in directionality mean? I think that in most of the world it is the horseshoe ITSELF that is lucky and protective -- whereas in England and Ireland the horseshoe is seen as a mere "collector" of luck from above. There are other regional and cultural differences in horseshoe beliefs, too.... 

Sorce: http://www.luckymojo.com/horseshoe.html