In slender io-stems the dative singular in Old-Irish ends in -iu. I find this form in the Annals of Ulster for the last time in a.d. 816 (Gertidiu). Thence onward it is always -i, as in our text (hi Cúailgni [43], d'uisci [64]).
The nasal stem léimm makes its nom. plur. léimmen in Old-Irish. In § [32] we find instead (tair-)leme. So also foimrimm makes its nom. plural foimrimme in § [163].
The word dorus is neuter in Old-Irish, making its nom. acc. plural either dorus or doirsea. In our text (§§ [173], [174]) the word is masculine, and makes its nom. plural doruis.
Druimm is an i-stem in Old-Irish, but in the later language passes into an n-stem. In § [51] we find the nom. pl. drommanna.
The neuter grád in § [166] makes its nom. plur. grúda for O. Ir. grád.[11]
[11] The infinitive bith for O. Ir. buith ([91]), the dative cinn for O. Ir. ciunn ([98], [135]), the nom. pl. sligthi for O. Ir. sligid (which I have restored in § [49]), the confusion between do and di (e.g. [83]), and other details are probably due to the Middle-and Modern-Irish transcribers.
On linguistic grounds, then, I should say that our collection was made some time during the second half of the ninth century. That it cannot be dated earlier is also apparent from another consideration. Professor Zimmer has taught us to search in every ancient Irish text for indications of its having been composed either before or after the Viking period. I find no words from the Norse language in the Triads, or, if there are any, they have escaped me; but there are two distinct references to the Viking age. In § [232], a Viking in his hauberk (Gall ina lúirig) is mentioned as one of three that are hardest to talk to; and, in § [44], Bangor in Co. Down is called unlucky or unfortunate, no doubt, as the gloss says, because of the repeated plunderings and destruction of its monastery by the Norse during the early part of the ninth century (a.d. 823, 824).
In endeavouring to trace the origin of the Triad as a form of literary composition among the Irish, one must remember that it is but one of several similar enumerative sayings common in Irish literature. Thus the collection here printed contains three duads ([124]. [133]. [134]), seven tetrads ([223]. [230]. [234]. [244]. [248]. [251]. [252]), and one heptad ([235]). A whole Irish law-book is composed in the form of heptads;[12] while triads, tetrads, &c., occur in every part of the Laws.[13] Such schematic arrangements were of course a great aid to memory.
[12] See Ancient Laws of Ireland, vol, v., pp. 118-373.
[13] Thus in the first volume of the Laws we find duads on p. 228, 15; 294, 27; triads on p. 50, 9. 27; 230, 4; 264, 20; 288, 28; tetrads 40, 21; 54, 7; 64, 1; 240, 24; 256, 4, &c.; 272, 25; 274, 3, &c.; pentads 30, 21; 50, 32; 90, 29; 102, 6; hexads 68, 11; 248, 7: a heptad 134, 9; an ennead 16, 20.