Kathy McCormack wrote:

> Can you explain why "Derb Forgaill" in "gnúis Derb Forgaill i
> l-luc lergg" is not genitive?  I'm assuming that "Deirbi
> Forgaill" a few lines earlier is genitive, but I could be wrong
> there!

You're not wrong.  They are both genitive forms of the name, as
demanded by context.  DIL agrees with this, s.v. "der".  In fact,
there appears to be a lot of confusion about what the proper
genitive might be.

Best & O'Brien, the editors of the Book of Leinster, title the
tale "Aided Derb Fhorgaill", while someone else (Marstrander?)
gave it the title you see in the subject line of this thread,
and they are both genitives.  A good deal of the problem seems
to hinge on how closely bound the two parts of the name are
felt to be.

BTW, you have raised some very interesting questions about the
mythological/narrative relationship of Derb Forgaill to Emer,
and about the possible playing out of a curse rooted in incest.
When we get a few lines further into the tale, CC's reason for
not marrying DF himself will further highlight the issue of
mating taboos.  We may need a chart to figure this out!  But
MacKillop does a good job of summarizing the goings-on in the
article "Lugaid Riab nDerg" in his _Dictionary of Celtic