Or geas; the injunction against some act or course of action, common in Old Irish story and perhaps belief. The ordinary scholarly comparison is to the Polynesian taboo, on which, however, I am the furthest thing from an expert; I am not prepared to speak to the accuracy of that comparison.
As for the geis, the idea of it is commonly misunderstood or misapplied in modern days as a curse or prohibition cast on someone by a magician; however, I cannot think offhand of any instance in the literature in which a druid (or anyone else, for that matter) places a geis on anybody, and certainly not at birth; indeed, this procedure would be absurd in many cases, such as that of Cú Chulainn's prohibition against dog meat. When the boy was young, he was called Sétanta; there is no indication then that he would have anything in particular to do with dogs. Even the word geis itself is misunderstood, in a way that illustrates the conceptual misunderstanding, for in contrast to modern stories like Clark Ashton Smith's The Seven Geases, it is never pluralized or even, so far as I can recall, used as a noun; rather, the common phrase is »that was geis to him«, an adjective.
Rather, the geis seems to be a thing that happens to a person; it is often tacitly understood, and is almost always appropriate somehow, even a consequence; for instance, and to return to the previous example, Cú Chulainn is forbidden to eat the flesh of dogs because after the incident with the smith's hound, he is now in some sense a dog; he has really taken on some part of the soul or nature of a dog by taking up its tasks, and to eat dog meat would thus be cannibalism, which it is not difficult to suppose was geis to any man. This type of infection by association is very common throughout the old Indo-European religions; I will restrict myself here to the single example of the horses used to pull the dead man's ship to the sea, in the funerary rites practiced at Old Uppsala. These horses were understood to have become liminal, tainted by contact with the world of the dead, and were consequently slain. (It was no small thing to kill a horse; they were far more expensive than men.)
In other words it is more consistent with the material to suppose that Cú Chulainn was previously just as able to partake of dog meat as anyone else, than that a sort of wizard would have bopped around his house when he was newborn, casually to doom him; this latter idea has more Tolkien than mythology in it anyway, and does not really reflect the operation of druidry in the Ulster Cycle, which is largely restricted to Cathub, and rather entails saying what will happen if a certain thing is done (typically in the format of »what the day is good for«: gaining lasting fame, conceiving a king (although this one may be a blatant lie), eating pie, whatever) than what is preordained in any case. In line with this, then, the druid's part (if any) is to identify the geis, in effect warning the hero what is not permitted for him, usually on pain of dire consequences — carrying on the running example, Cú Chulainn's eventual breaking of the dog-meat geis leads to his death — but on occasion it appears that breaking the geis is just an offensive and antisocial behavior, which seems likely to be closer to its role in real Old Irish society. (As an example of this latter, see Cú Chulainn driving the wrong way around Emain, which appears to have no real consequence, and just constitutes thumbing his nose at the inhabitants.)
Notwithstanding these occasional »harmless« geasa, it's clear that it was seen as vital for the heroes of Old Irish literature to know the nature of any geis they might be under — we might say their limits in life — and it also appears that accruing more of them was a fairly inevitable side effect of a life of heroics. Notice that this conception of a kind of fate operating in reciprocity with free will, a form of cause and effect much like that which we are all used to, only treating with a hidden world, fits far better with the idea of a man choosing to act in a way that gives him an affinity with dogs and consequently certain associated restrictions, than with the Greek type of intractable doom.