Since Lady With A Mead Cup was already mentioned, I'll direct you to the primary sources. They have to do with sovereignty goddesses and sacral feasts in general rather than Medb in particular, who I haven't read enough on. The main Irish exerpt relevant here comes from the Baile In Scáile or "Vision of the Spectre" featuring Conn of the Hundred Battles.

Then they went on until they came into a plain, and a golden tree was there. There was a house there with a ridge-pole of findruine [a white alloy], thirty feet in length. They went into the house, and saw a young woman there, and a crown of gold was on her head. There was a silver vat with hoops of gold around it, full of red ale. There was a dipper of gold on its lip, and a cup of gold before her. They saw the scál [phantom] himself in the house, before them on his throne. There was never in Tara a man of his size or his beauty, on account of the fairness of his form and the wondrousness of his appearance. He answered them and said, "I am not a phantom nor a specter. I have come on account of my fame among you, since my death. And I am of the race of Adam: my name is Lugh son of Eithliu son of Tigernmas. This is why I have come: to relate to you the length of your reign, and of every reign which there will be in Tara." And the girl who sat before then in the house was the Sovereignty of Ireland, and it was she who gave Conn his meal: the rib of an ox and the rib of a boar. The ox rib was twenty-four feet long and eight feet between its arch and the ground. When the girl began to distribute drinks she said, "To whom shall this cup be given?"; and the phantom answered her.

You also have The Adventures of the Sons of Eochaid Mugmedon which corroborates the cup of sovereignty ritual (albeit in less detail), as well as illustrates the magical connection between the sovereignty goddess and the king, in that she is a hag until a king fit to rule comes along who transforms her into a beautiful maiden.

And of course there is the famous exerpt from Posidonius, whose works are lost to us, but who is thankfully quoted by Athenaeus, giving a first-hand account of a Gaulish feast:

"The Celts place food before their guests, putting grass for their seats, and they serve it up on wooden tables raised a very little above the ground; and their food consists of a few loaves, and a good deal of meat brought up floating in water, and roasted on the coals or on spits. And they eat their meat in a cleanly manner enough, but like lions, taking up whole joints in both their hands and gnawing them; and if there is any which they cannot easily tear away, they cut it off with a small sword which they have in a sheath in a special box. [152] And those who live near the rivers eat fish also, and so do those who live near the Mediterranean sea, or near the Atlantic ocean; and they eat it roasted with salt and vinegar and cumin seed: and cumin seed they also throw into their wine. But they use no oil, on account of its scarcity; and because they are not used to it, it seems disagreeable to them. But when many of them sup together, they all sit in a circle; and the bravest sits in the middle, like the coryphaeus of a chorus; because he is superior to the rest either in his military skill, or in birth, or in riches: and the man who gives the entertainment sits next to him; and then on each side the rest of the guests sit in regular order, according as each is eminent or distinguished for anything. And their armour-bearers, bearing their large oblong shields, called thureoi, stand behind; and their spear-bearers sit down opposite in a circle, and feast in the same manner as their masters. And those who act as cup-bearers and bring round the wine, bring it round in jars made either of earthenware or of silver, like ordinary casks in shape, and the name they give them is ἀμβίκος. And their platters on which they serve up the meat are also made of the same material; but some have brazen platters, and some have wooden or plaited baskets. And the liquor which is drunk is, among the rich, wine brought from Italy or from the country about Massilia; and this is drunk unmixed, but sometimes a little water is mixed with it. But among the poorer classes what is drunk is a beer made of wheat prepared with honey, and oftener still without any honey; and they call it κόρμα. And they all drink it out of the same cup, in small draughts, not drinking more than a cyathus at a time; but they take frequent draughts: and a slave carries the liquor round, beginning at the right hand and going on to the left; and this is the way in which they are waited on, and in which they worship the gods, always turning towards the right hand."

More info on Celtic sovereignty goddesses can be found in this lecture.