The Táin Bó Cúalnge has been preserved, more or less complete, in a score
of manuscripts ranging in date from the beginning of the twelfth to the
middle of the nineteenth century. There probably existed other manuscripts
containing not only the Táin as we have it but even episodes now wanting in
it. All of the extant manuscripts go back to versions which date from the
seventh century or earlier. No manuscript of the Táin is wholly in the
language of the time when it was copied, but, under the cloak of the
contemporaneous orthography, contains forms and words so obsolete that they
were not understood by the copyist, so that glossaries had to be compiled
to explain them.

It is by a singular good fortune that this, the greatest of all the epic
tales of the Irish, has been handed down to our day in the two most ancient
and, for that reason, most precious of the great Middle Irish collections
of miscellaneous contents known as the _Leabhar na hUidhre_, "the Book of
The Dun (Cow)," and the Book of Leinster. The former and older of these
vellum manuscripts (abbreviated LU.) is kept in the Library of the Royal
Irish Academy at Dublin. It must have been written about the beginning of
the twelfth century, for its compiler and writer, Moelmuire macCeilechair
(Kelleher), is known to have been slain at Clonmacnois in the year 1106;
some of its linguistic forms, however, are as old as the eighth century
glosses. Unfortunately, LU.'s account of the Táin is incomplete at the
beginning and the end, but the latter portion is made good by the closely
related, though independent, version contained in the manuscript known as
the Yellow Book of Lecan (abbreviated YBL.). This manuscript was written
about the year 1391 and it is also kept in Dublin in the Library of Trinity
College. To the same group as LU. and YBL., which for the sake of
convenience we may call version A, belong also the British Museum MSS.,
Egerton 1782, a large fragment, and Egerton 114, both dating from the
fifteenth or sixteenth century.