Material
Cold iron — the one thing British and Irish folklore agrees about more than almost anything else.
These are associations the community holds, not settled fact — the more a reading is shared, the bolder it sits.
Iron against the otherworld is remarkably consistent across British and Irish folk belief: nails driven into a threshold, a horseshoe over the door, scissors laid open under a mattress, a knife by the bed of anyone thought to be vulnerable. Frazer collected the mirror image as well — priests, chiefs and sacred kings in many places forbidden to let iron touch them at all. A substance can be both a ward and a taboo, and iron very often is both in the same district.
Agrippa puts iron under Mars, with everything hot, harsh and quick that follows from it; Culpeper's herbal runs the same planetary machinery on plants. That reading has been stable for centuries and still sits underneath modern practice, where iron is used for protection, for holding a boundary firm, and for cutting.
Common forms are what you would expect: nails, a horseshoe, scissors, a knife, filings for a bottle, a cast pan handed down. Opinions on which way up a horseshoe belongs are furious and unresolvable. Old nails are sharp and rusty — clean any cut properly, and know whether your tetanus cover is current.
Readings practitioners have offered, ordered by how many recognise them. Take what serves, leave the rest.
Carried forward
James Frazer, The Golden Bough (1890)
Frazer sets the two halves of the iron rule side by side: nails and horseshoes used everywhere to keep spirits out, and sacred persons in many of the same regions forbidden to be touched by iron at all. His explanation is that spirits and holy people were held to share a dislike of a new and unnatural metal — and he is careful to call that a guess.
Carried forward
Common in contemporary practice
A single nail set at a threshold and left there, rather than a whole apparatus. The advice most often repeated is that a ward you would be embarrassed to explain to a visitor is a ward you will eventually take down.
Credit what you carry. Much of this comes from living traditions, and some of it from traditions that are closed — held by particular peoples and not offered for general use. Where you know whose practice a reading came from, name them; where you don't, say that too.