Fairies - Protection Against
Cross - From the earliest days of Christianity the cross was believed to be a most potent protective symbol against fairies and all evil spirits. It is even possible that cross-roads had a pre-Christian significance, as sacred to the god of limits and a place of sacrifice. The cross in all its forms was protective - the 'saining' or crossing of one's own body or that of another, a cross scratched on the ground or formed by four roads meeting, a cross of wood, stone or metal set up by roadside, a cross worn as a trinket round the neck, all these were believed to give substantial protection against devils, ghosts or fairies. Sometimes this protection was reinforced by carrying a cross of a particular material - of rowan wood, for instance, for this wood was a protection of itself - or for trinkets crosses of coral or amber, both of some potency.
Iron - Iron is the traditional bane of faeries. Some of the folk can not bear to touch it, while others will actually be forced away at the sight of cold iron such as a horse shoe. All faerie tools and weapons are made of stone (in the case of the common flint arrowheads, or elf-shot), wood or less commonly bronze. This fear might also explain the marked decrease of faerie sightings since the industrial revolution.
"all uncouth, unknown wights are terrified by nothing earthly so much as cold iron"
Iron nails on the door would ward off faeries from a household, while placing a piece of iron in a baby's crib will keep a changling from taking its place. When entering into faerie mounds, sticking an iron knife in the doorway will ensure your escape. Wearing an iron cross, is a doubly effective ward against the wee folk since it uses both iron and religion . Many believe that the fear of Iron dates back to when the Celts arrives in Britain with their iron weapons, with these they easily displaced the bronze and stone using peoples who occupied Brtiain at these times