Part of Psalm 4:5 from PUL's translation:

Na nithe adeirthi ' núr gcroithibh deinidh aithri iontu ar úr leapachaibh

The english he translate it from says:
"Reflect, upon your beds, in silence"

Mainly the beginning is throwing me off... I can't seem to find the translation of "nithe", "adeirthi" and "núr gcroithibh"

I appreciate all explanations!! THanks

nithe is the plural of ní 'thing'. I believe adeirthí is an alternative from for a deireann sibh 'that you (plural) say'. núr gcroithibh is in bhur gcroí 'in your (plural) hearts'. Literally it is: 'the things that you say in your hearts, make (deinidh is the imperative of dein/déan) penance in them on your beds (leapachaibh is the dative plural of leaba(idh)).

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I'm an intermediate speaker of the Corca Dhuibhne dialect of Irish and also have knowledge on the old spelling
Soir gaċ síar, fé ḋeireaḋ thíar

na nithe = the things (sing. ní = thing)
adeirthí = a deir(eann) sibh = which you say (-thí is the old present tense 2nd person plural form I thought to be obsolete)
’núr = in bhur = in your
gcroithibh = hearts (dative plural ending -ibh), a fada is missing (CO croí, croíthe), the i is probably short before plural ending -the, -thibh, as well as in ní, nithe, nithibh.
deinidh = déanaigí = make (ye) (imperative 2nd person plural)
aithrí = penance
iontu = in them (for the things)
ar úr = ar bhur = on your
leapachaibh = beds (again dative plural)

Nobody in Kerry, or outside of Munster uses any other form but the analytical ending with sibh. I would also be very surprised if anyone still used this today in Cork.

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I'm an intermediate speaker of the Corca Dhuibhne dialect of Irish and also have knowledge on the old spelling
Soir gaċ síar, fé ḋeireaḋ thíar