(or Gessa, plur. for Geis) Singular: Geas(gaysh), plural: Gease(gaysha).
Caesar said the goose was sacred to Celtic tribes and was not considered edible, because of her connection with the Sun-Egg. For similar reasons, medieval superstition forbade the killing of a goose in midwinter, when the sun was thought to be in need of maternal care to gain strength for the new seasons. Like other formerly sacred creatures, geese were said to contain souls of the unbaptized (pagans).It was associated with both Celtic and
Teutonic war gods, who were accompanied by a horse and a goose. In Gallic iconography epona, The Divine Horse, is depicted riding on a horned goose.
The Norse did not eat the goose.

A bronze figurine of a warrior-goddess at Dinéault in Brittany depicts her wearing a helmet with a goose crest. The Celtic Mars was associated with geese: he appears thus at Risingham in North Britain; and a goose was the companion of MARS THINCSUS (a Germanic deity) at Housesteads. The bird
accompanies the peaceful healergod Mars Lenus at Caerwent, presumably being

The violation of Gease  is such a sure omen of approaching death that it might almost be inferred that a hero is safe from harm while his gease remain inviolate. Then, as his time approaches its end, he finds himself in situations where he cannot avoid breaking them, just as Greek heroes unwittingly work their own undoing when their fated hour has come and their divine guardians have forsaken them. Nowhere is this process so dramatically depicted as in ‘The Destruction of Da Derga’s Hostel, where in the course of the events which lead up to his death Conaire violates one after another the gease laid upon him, by the King of the Birds, before he was installed King of Ireland. These gease were:

1. Thou shalt not go right-handwise round Tara and left-handwise round Brega.
2. The crooked beasts of Cerna must not be hunted by thee.
3. Thou shalt not be away from Tara for nine nights in succession.
4. Thou shalt not stay a night in a house from which firelight can be seen after sunset and into which one can see from outside.
5. Three Reds shall not go before thee to the house of Red.
6. No plunder shall be taken in thy reign.
7. After sunset a company of one woman or one man shall not enter the house in which thou art.

The law of the geas(or geis): The tale of Conary introduces us for the first time to the law or institution of the geis, which plays henceforward a very important part in Irish legend, the violation or observance of a geis being frequently the turning-point in a tragic narrative. We must therefore delay a moment to explain exactly what this peculiar institution was. Dineen’s ‘Irish Dictionary’ explains the word geis as meaning ‘a bond, a spell, a prohibition, a taboo, a magical injunction, the violation of which led to misfortune and death,’ (The meaning quoted will be found in the Dictionary under the alternative form geas). Every Irish chieftain or personage of note had certain geise peculiar to himself which he must not transgress. These geise had sometimes reference to a code of chivalry – thus Dermot of the Love-spot, when appealed to by Grania to take her away from Finn, is under geise not to refuse protection to a woman. Or they may be merely superstitious or fantastic – thus Conary, as one of his geise, is forbidden to follow three red horse-men on a road, nor must he kill birds (this is because his totem was a bird). It is a geis to the Ulster champion, Fergus mac Roy, that he must not refuse an invitation to a feast; on this turns the Tragedy of the Sons of Usnach. It is not at all clear who imposed these geise or how any one found out what his personal geise were – all that was doubtless an affair of the Druids. But they were regarded as sacred obligations, and the worst misfortunes were to be apprehended from breaking them. Originally, no doubt, they were regarded as a means of keeping oneself in proper relations with the other world – the world of Faery – and were akin to the well-known Polynesian practice of the ‘tabu.’ Rolleston prefer, however, to retain the Irish word as the only fitting one for the Irish practice.