The construction and arrangement of the tales were carefully studied by the Irish literary men of the olden time, and not more than their importance deserved. They were arranged in seventeen classes or groups, and in each group there were a number of individual stories. This grouping was a great help to the storyteller, who had to store up in his memory a large number of tales: for by having them in this manner, sorted as it were in parcels, he was enabled to call them up all the more readily--to put his hand on them, so to say--when he wanted them. ‘Voyages,’ for instance, formed one group, which included “The Voyage of Maeldune,” “The Voyage of St. Brendan,” “The Voyage of the Sons of O’Corra,” and many others. Another was ‘Tragedies,’ under which came “The Fate of the Children of Lir,” “The Fate of the Sons of Usna,” etc., etc. There were ‘Military Expeditions,’ ‘Courtships,’ ‘Cattle-raids,’ ‘Sieges,’ and so on, to the number of seventeen, each group with its own parcel of stories. We have in our old books stories belonging to every one of these classes. The whole number now existing in manuscripts is close on 600: of which about 150 have been published and translated. But outside these, great numbers have been lost: destroyed during the Danish and Anglo-Norman wars. Most of the Irish tales fall under four main cycles or periods of history and legend, which, in all the Irish poetical and romantic literature, were kept quite distinct. _First_:--The Mythological Period, the stories of which are concerned with the mythical colonies preceding the Milesians, especially the Dedannans. The heroes of the tales belonging to this cycle, who are assigned to periods long before the Christian era, are gods, namely, the gods of the pagan Irish. _Second_:--The Period of Concobar mac Nessa and his Red Branch Knights, who flourished in the first century. These Red Branch Knights were a sort of heroic militia, belonging to Ulster, mighty men all, who came every year to the palace of Emain to be trained in military science and feats of arms, residing for the time in a separate palace called Creeveroe or the Red Branch. Their greatest commander was Cuculainn, a demigod, the mightiest of all the Irish heroes of antiquity, whose residence was Dundalgan, now called the Fort of Castletown, near Dundalk. Others of these great heroes were Conall Kernagh, Laery the Victorious, Keltar of the Battles, Fergus mac Roy, and the three Sons of Usna--Naisi, Ainnle, and Ardan. They were in the service of Concobar or Conor mac Nessa, king of Ulster, who feasted the leading heroes every day in his own palace.