In Irish mythology the shades of departed heroes were frequently summoned to give account of themselves. When King Mongan of Ulster was in dispute with the bard Forgoll, who had threatened to satirise the king’s family over the question of the true account of certain events in the story of Fionn Mac Cumhail, the Fenian hero Caoilte returned from the Otherworld and substantiated Mongan’s claim. On another occasion, when Senchan Torpeist, the Chief Poet of Ireland in about the year AD 598 called a meeting of bards and story-tellers to see if they could remember the whole of the great epic poem Tain bo Cuailgne [The Cattle-Raid of Cooley] (155), he found they could recall only fragments and forthwith sent two young poets forth in search of an ancient, long lost book said to contain the whole of the Tain, and which had apparently been given away in exchange for a copy of Isadore of Sevilles Culmen.
On their way they reached the tomb of Fergus Mac Roith at Magh Aei in Roscommon, and there one of the young bards recited a poem of his own. At once he found himself enveloped in a thick mist, in which the figure of Mac Roith himself appeared and proceeded to recite the entire Tain over a period of three days and nights.
This is fairly clearly a reference to a kind of inspired shamanic trance in which the bard was able to communicate with the great dead and retrieve lost knowledge and understanding of his people’s tradition. (That Taliesin apparently has access to all wisdom gives an idea of the respect in which he was held.)
Another, Christianised, version of the above story tells how the assembled saints of Ireland prayed at the tomb of Mac Roith, and that he appeared to St. Ciaran of Clonmacnoise, who wrote the entire poem on vellum manufactured from the skin of his favourite dun cow. The book thus constructed was thereafter named Leabhar na hUidre, ’The Book of the Dun Cow’, copies of which still exist today (59). What is  interesting about this story is not only that it shows the Christian monks recognising the Otherworld as a source of knowledge unobtainable by any other method, but also because the idea of recording the epic on the skin of a cow recalls the method of the incubatory sleep which bards used to take wrapped in a bulls hide in order to receive messages from the Otherworld.
Celtic literature abounds in such inspiration raids on the Otherworld, and a fuller explanation of these must wait for another occasion. For the moment we might mention the Voyage of Bran Mac Febel to the Land of Promise (209) where, having received the Silver Branch (which, a we shall see, is the sign of all the poets), Bran sails off to a series of fantastic adventures which, on his return to the realm of mortals, he recounts in a series of Ogham quatrains. After which he bids them farewell, and from that hour his wanderings are not known. (pp.112-13.)