Breandán wrote:
If it's "a stain of red blood", wouldn't that be
smál fola deirge ? (Or using Sean79's
corcairdhearg,
smál fola corcairdheirge ?) because the adjective is genitive singular feminine (gsf).

Yup.
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On the other hand, if the
dearg is modifying the whole phrase, then it wouldn't need to be lenited as the core of the phrase
smál is accusative singular masculine (which is the same as nominative singular masculine in Irish). However, in that case, Sean's positioning of the adjective would be clearer, i.e.,
smál dearg fola or
smál corcairdhearg fola.

Yup, on both accounts, though if
smál fola is perceived as a single word, the adjective might just as well follow, similar to how ‘a young student’ will always be
mac léinn óg, never
*mac óg léinn.
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I'm leaning towards smál fola corcairdheirge myself, but are there reasons one of the other versions is better? (or reasons that my analyses were incorrect?)
I think the only problem with this is that blood is not usually crimson in Irish: it’s just red.
Corcairdhearg is really just a concise description of a colour (‘purple-red’)—it doesn’t have the dramatic connotations of the English word. It’s kind of like choosing a nuance of crimson in English and applying that to blood: ‘an alizarin blood stain’ doesn’t really sound right, either, for example.