Horse Shoe. For some They're Lucky...
In Irish folklore there is much ado made of the powers of a Blacksmith. We see this in the high regard Goibhniu holds within the Tuatha Dé Danann to the deference that King Conchobar makes towards Culainn the smith in the Ulster cycle. The knowledge of fire held by these people, and the skill to use said fire to shape some of the strongest elements has long been held in high regard.
These individuals are often imbued with an almost kingly reverence for their talent, which many of our ancient ancestors viewed as a form of arcane mysticism. Indeed it was said that the weapons made by the smith God Goibhniu that "No spearpoint which my hand shall forge, shall make a missing cast. No skin which it pierces shall taste life afterwards.' and that he would replace every weapon which may break, for the warriors of the Tuatha Dé Danann, even were they to battle for seven years, during their conflict with the Fomorians.
One of the very common items which made the move from practical to folkloric value was not a weapon though. It was a horse shoe. For some the value of the luck or protection came from the magic of the smith themselves, some of which were known to have a healing charm upon them, or the power to wed a couple in a stable and bountiful union.
For others its the tale of the blacksmith avoiding the trap of the devil, by shoeing his hooves with red hot nails and casting away the money the devil gave him. The devil suffered terrible pain from the shoes and is said to have cast them off and been shy of going near horse shoes since.
What is also known from the tales is that forged iron is anathema to the sidhe, or folk of the Otherworld. Whether it is because it comes from below the earth where some are said to reside, or because it is the most stable metal created with such heat that the memory of that heat is forever captured within it. Iron, the chief component of horse shoes, is said to cause some of them great pain should they be touched with even the smallest part of it, or at the very least it would cast off their glamours and reveal them for their true selves.
To this day you will still find horse shoes hung above doorways as a sign of luck, and there are many tales of the proper way that it should be hung to ensure that ones luck would not drain out of it.
Whatever your belief, it could be fair to say of a horse shoe that, for some it is lucky, and for Others, its an allergy.