Remember that I supplied a James Macpherson link specifically to
an excerpt from something called Darthula, in an early post?

Darthula turns out to be Macpherson's renaming of no less than
Deirdre of the Sorrows, for heaven's sake.
Some connection to the name "Dearshul," Thomson supposes (page 51).

This "Ossian" re-writing merits its own section of commentary in
Derick S. Thomson's Gaelic Sources.
The author does not mince words.

quote
The Story of Deirdre is one of the oldest in Scottish, as distinct from Irish, tradition.
A prose version exists in the Glenmasan MS., which may be a copy
of a MS. from the first half of the thirteenth century.
[Footnote:
from the Catalogue of Gaelic MSS. by prof. D. MacKinnon:
'This MS. does not date further back than 1500, but
it may well be a copy of an older one of date 1238'.]

...Macpherson's story bears only a general relation to the traditional one [of Deirdre of the Sorrows],
and it has been adapted to fit in with Macpherson's own scheme of history.
Names are altered at will ...
Cuthullin [Cuchullainn] takes the place of 'Conchobar' as the uncle of the three brothers ...
The home country of [the brothers' father Uisneach] is represented as around Loch Eta in Argyllshire [!].
This is the Loch Etive associated with Deirdre in Scottish tradition, but
Macpherson is here shifting the original habitat of the main characters -- as distinct from their place of sojourn -- from Ireland to Scotland.

...Macpherson may be said to adapt his sources with some ingenuity, but in so doing he loses much of the story.
The representation becomes blurred.
The fine clear colours of the original are gone --
we no longer see the red blood and the black raven against the whiteness of the snow.
At times, indeed, the course of the story becomes hard to follow.
In this telling the tale has lost its tragedy, its pathos, its dignity, and practically all of its meaning.
endquote

from Chapter V, pages 53 - 55, excerpted