These are a few -- disorganised -- notes on the tale given on this site. Read it if you like, ignore it if you prefer, but give no weight to any of the points made if you disagree with them. I make no pretence to any scholarship of Irish or Scottish Myth, nor to any knowledge of Gaelic story-telling tradition.
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The tale of Deirdìre of the Sorrows is the last, and probably the greatest, of the Three Sorrowful Tales of Erin (the other two being The Fate of the Children of Turenn and The Fate of the Children of Lir). Like any tale passed down in oral tradition, many versions exist, and there is no definitive source for it. However, early medieval manuscripts such as the Book of Leinster give the tale in a form very close (though with interesting differences) to the one recorded by Alisdair Carmichael in Barra in the late nineteenth century.
Many of the trappings of the Irish mythological cycles are stripped away in the Barra version of the tale. Cuchullain, the Hound of Ulster, is not mentioned, although in other versions he is one of the heroes Conachar tries to send to Alba to persuade Naoise to return. Conachar the King and Duanan Gacha Draogh, his druid, are not responsible for initiating events as they are in the Book of Leinster; instead, a nameless, mendicant soothsayer delivers the prophecy that causes Colum Cruitire (Deirdìre's father) to hide his daughter away. Colum is no longer storyteller to the king -- although an echo of his rôle is apparent in his name, Colum the Harper. And instead of being sent away to a king's fortress, the infant child is brought up in a simple green mound far away from any human habitation.
Possibly this is a consequence of skillfull storytellers relating their material more closely to the daily experiences of their audience. There is a clear shift away from anything courtly or formal -- palaces, castles and banquests -- to simple halls and wide countryside. Even though Conachar's palace is mentioned several times, a description of it is never attempted. Contrast that with the many passages describing (in almost baroque style) the natural world.
The other (minor) difference which you will at once notice is the spelling of the main character's name: Deirdìre instead of the Deirdre of tradition. The stress -- at any rate when I heard it spoken -- was on the last syllable and I have added an accent to the terminal "i" to indicate this. To my ears the Barra pronounciation is much to be preferred, purely for its mellifluence. (Although it may make clearer the relationship between the word Deirdìre and the Gaelic word for tears - "deuran" -- which is in a sense the main theme of the tale: "Deirdìre a fras-sileadh na deuran".)
The Victorians, with the considerable interest in Celtic tales Sir Walter Scott's Waverley novels had awakened in them, were quick to latch onto the Book of Leinster version of the tale, calling Deirdìre the "Irish Helen" and (rather ridiculously) refering to the Laoidh Chlann Uisne as the "Irish Iliad". But in truth there are many parallels and echoes in the story -- for this folktale, like all folktales, contains elements that mysteriously appear in oral tradition from all around the world.
The mysterious stranger announcing to a disbelieving (and elderly) man that he would have a child recalls Abraham's meeting with God (Genesis 18). Indeed, the second chapter of the story in particular has several explicit (and presumably, in strict logic, anachronistic) references to God -- "O, mhuime, dh' iarr an t-ian a stigh air sgàth Dia-nan-dùl, agus their thu fhèin riumsa nì air bith a dh' iarrar oirnn na ainm-san gur coir dhuinn a dhèanadh." -- although the last chapter is entirely pagan, and the more tragic (like King Lear) because of it.
Such Biblical references or echoes may be put down to the influence of Christianity on the storytellers. A far older echo (familiar to us from the Oedipus myth) is the attempt by characters in a drama to avoid a terrible prophecy, and by that very act to bring it about. And the tale of Deirdìre, like most pagan Irish (and Norse) myth is always very concerned with the hopeless (but nonetheless worthwhile -- and in the poetic and moral sense -- beautiful) striving of mortals against an inexorable, immovable fate. We can hear it (chapter three) in the simple (but marmoreal) "agus thill Naois": three short, stressed syllables, but with the entire weight of a tragic destiny sloting into place behind them.
Comparisons with Helen of Troy are not especially illuminating. But, like Homer, the storyteller is careful to describe the impossible, incredible beauty of Deirdìre by its effect on others. The hunter, faint with hunger and thirst beforehand, feels the need for neither after seeing her face. Gealbhan Greadhnach tells Conachar that even though Naoise put out his eye he would have continued to gaze at her with the other, if he could. And of course there are obvious parallels between Menelaus and Conachar (who is, however, more like Agamemnon in his vicious ruthlessness).
While it would be misleading to call the story of Deirdìre a fairy-tale -- it is much nearer to what Northrop Frye calls "Romance"; that is, it tells us of characters that are superior in degree, not kind, to other men and their environment -- it contains many things traditionally associated with that genre. For instance, although there are no daoine-sìthe in the world of this story, Deirdìre's hiding place is very clearly modelled on a fairy mound. Perhaps her great (perilous) beauty suggested something of the fairy-maiden to the Scottish story-tellers.
The ubiquitous use of "threes" in the tale is also a staple of fairy-stories. The hunter calls three times before he is let into the mound, as does Conachar the King. Deirdìre tries to dissuade Naoise from returning to Eirinn three times as well, in verse of strikingly vivid imagery (though the third time is more a lament than a plea). Partly this is a function of the mechanics of oral retelling, but it can be a very effective dramatic technique. Two of the sons of Fearchair Mac Ro go out, fight for a while, then accept the King's bribe. So the sudden, spirited loyalty of Fiallan Fionn amongst so much betrayal ("An ta, Chonachair, cha ghabh mi an tairgse sin uait no taing air a shon.") is all the more effective. The apotheosis of this technique is seen in the last, desperate struggle of the brothers and Deirdìre through the ice-choked sea.
In the tale of Deirdìre you will find betrayal, and unlooked-for loyalty; vindictive hatred, and love that is faithful to the very edge of the grave. And though it is undisputably full of tradgedy, sorrow and tears there is still a deep seam of solemn joy through it, that says no matter how black fate is, it is better to face it down than flee from it.