The physician of the Irish gods is best remembered for a murder, and for destroying the most valuable thing anyone in the tradition ever produced.
The arm, and the better arm
When Nuada loses his hand in the first battle of Mag Tuired, Dian Cécht fits him with a replacement of silver, working with Credne the brazier, who supplies the rivets. The result moves as well as any other arm, and it is enough to earn Nuada the epithet Airgetlám and not enough to restore him to the kingship, because Irish law wants a body without visible defect.
His son Miach then does better. In the fuller account he goes to Nuada's household, finds the silver arm in place, and grows flesh over it — or replaces it entirely — reciting an incantation about sinew joining to sinew and nerve to nerve, and in three periods of nine days the arm is a living arm again.
Dian Cécht's response is the passage everyone remembers. He strikes his son on the head with a sword, cutting the skin; Miach heals it. He strikes again, to the bone; Miach heals it. A third blow opens the membrane of the brain; Miach heals that too. The fourth cuts the brain itself, and Miach dies, because no physician can heal that.
The text does not soften it, does not have him regret it, and does not have anyone punish him. A father kills a son for being the better surgeon.
Airmed's cloak, and the loss of the whole pharmacopoeia
What follows is worse in its consequences. From Miach's grave grow three hundred and sixty-five herbs, one for each sinew and joint of his body, each with the power to cure the part it grew from. His sister Airmed gathers them and lays them out on her cloak, arranged according to their properties — the first materia medica, sorted and complete.
Dian Cécht comes and overturns the cloak, mixing them beyond recovery, so that nobody knows their proper uses. The text adds a line that is one of the most quietly devastating things in Irish literature: unless the Holy Spirit were to teach them afterwards.
Two readings are available and neither is decorative. It can be taken as an aetiology — an explanation for why medicine is difficult and knowledge incomplete, of the kind many cultures produce. It can also be read as a monastic scribe's comment on the limits of pagan learning, since the escape clause is Christian. Both are probably operating at once.
The Well of Sláine
His constructive moment comes in the second battle. Dian Cécht, with his sons Octriuil and Miach and his daughter Airmed, stations himself at a well called Sláine — health — and chants over it, so that mortally wounded warriors of the Tuatha Dé Danann put into it that evening come out whole the next morning and fight again.
The Fomorians work out what is happening and destroy it in the most practical way available: every man carries a stone from the river and drops it in until the well is filled and a cairn stands where it was. It is a rare moment of the enemy behaving like an army rather than like monsters, and it is the reason the battle can be lost at all.
Note that the healing is verbal. The four physicians recite over water; the water does the work. That is closer to how early Irish medical charms actually operated than to surgery, and it sits oddly beside the very physical business of the silver arm and Miach's grafted flesh.
The serpent in the boy's heart
A place-lore poem gives him a different kind of intervention. Meiche, a son of the Morrígan, had three hearts, and in each was a serpent that would have grown to lay Ireland waste. Dian Cécht killed him, burnt the hearts, and threw the ashes into a river — which boiled at the contact and killed everything in it, and is named from that.
Preventive destruction on a national scale, performed by the physician. The doctor in this tradition is not a gentle figure.
A law tract with his name on it
The most striking thing about Dian Cécht is not in the mythology at all. Early Irish law includes a substantial tract called Bretha Déin Chécht, the Judgements of Dian Cécht, dealing with injury: how wounds are to be assessed, what compensation is due for damage to each part of the body, the physician's obligations, sick-maintenance, and the fee structure for treatment. It is a real eighth-century legal text of a highly technical kind.
Whatever else he was, his name was attached to the professional standards of Irish medicine by people whose job was assessing broken bones for money. That is a more concrete trace than most figures in this tradition leave, and it is worth more than any number of modern lists of his supposed domains.
Was he a god of healing or something narrower?
The material consistently shows him as a practitioner rather than a patron. He does procedures, he supervises staff, he defends his professional position violently, and a body of law carries his name. Nothing describes him receiving prayers from the sick or presiding over health in the abstract. Calling him the Irish Asclepius imports a cult that Ireland does not evidence — though the incubation shrine at Lydney, in Britain and in Latin, shows the Greco-Roman model operating on Celtic-named ground.
Why does the herb story specify 365?
The number is the count of joints and sinews attributed to the human body in the same medieval learned tradition that gives the year 365 days, and the correspondence is deliberate: one remedy per part, a complete system, and the year mapped onto the body. It is a scholarly conceit rather than a botanical claim, and it signals that what Airmed lost was total knowledge rather than a large collection.
Does Airmed have a following of her own?
She has almost nothing in the medieval texts beyond this episode, and a great deal in modern herbalism and pagan practice, where she has become a patron of herb-craft with attributes assembled in the last few decades. The one genuinely medieval thing about her is the image of the sorted cloak, which is a remarkably good emblem for the discipline — knowledge is the arrangement, not the specimens, and it is the arrangement that was destroyed.