I'm not at all sure how to put this into Irish - again it's taken from the storytelling of Séamus Ennis and again I have the feeling that it's Irish transposed into English.
- "I'll have the head down off you by sunset"

Here's the context -

And the next morning, Féidlim Tonn Rí comes in, very early
and he said "What you have to do today,
you have to clean out an old byre that's here,
that hasn't been cleaned for a long number of years.
And you have to clean every morsel, everything,
leave it as clean as a new pin as the saying is.
And what's more you have to find my Grandmother's cambric needle,
which got lost in that byre so many years ago.
And if you don't have that done,
I'll have the head down off you by sunset."

cuirfidh mé an ceann síos asat roimh luí na gréine - would that make any sense or not?

I'll have the head down off you by sunset.

I'd say -
Bainfidh mé an cloigeann/ceann díot roimh luí na gréine.

Go raibh maith agat - that makes much more sense, and after seeing that, it always seems so obvious.

And for the word - morsel - which I always thought meant a small piece of food, but obviously here it is being used to mean any sort of small object or amount of something, I've found several words in Irish, but I'm not at all sure which would be the more appropriate one.

- ruainne (as in - ruainne beag bídeach)
- cáithne and the diminutive cáithnín
- blúire (as in - gach aon bhlúire)
- smut