Here's my word-for-word take on it:

> Druidheacht is îdlacht, maith,

magic and idolatry, good(ness)* (= magic?),

> in ailc mîn-glan mûr-glan,

in ____** smooth-clean wall-clean

> bârc dîbeirgi, dûain gil,

book of-wrath***, poems bright,

> is ûaidib ro·mûnadh.

it-is from-them has-been-taught

* "Maith" is usually unambiguous, but here I'd be inclined
to follow Liz's suggestion, drawing on Dinneen.  "Maith"
meaning "magic" might have been a noa term, naming what you
fear by its opposite.

Also, we have this use of "máeth n-áirig" in the LL Táin,
literally a "máeth of binding, guaranteeing":

> Is and sain rasiacht Medb máeth n-áraig bar Fer nDiad im
> chomlond & im chomrac [...]  Rasiacht Fer Diad máeth
> n-áraig furri-si ...

** The translation given for "ailc" is "house", which I
don't recognize.  I'm puzzled, and all I can suggest right
now is to read "ailc" as "áirc = chest, coffer".  There is
also an example of "ailc" in bérla na filed meaning "feeble",
but I don't see how that would fit.

*** The genitive sg. of "díberg" is "díbe(i)rge", with
typical Middle Irish confusion of the final vowel.  The word
originally means "marauding, pillaging", but later takes on
the extended meanings of "wrath, destruction, injury".

The sense I get is that these druids taught from "a destructive
book" which contained "bright poems", which may have been
housed in a "polished chest".

> Môradh sred is mana,

was-exalted sneeze and portent

> raga sin, am sona,

choice of-weathers, time lucky (= lucky time),

> gotha én do faire

voices of-birds to watch (i.e. pay attention to)

> [c]airi gach ceôl cona.

heed of every song of hounds**.

** The translation given for this line is "They practiced
without disguise", which I don't get at all.  I've amended
"cairi" to "aire", and I read "cona" as "cúana", gen. pl.
of "cúan = pack of dogs, wolves", or else as a late gen.
pl. of the word "cú" itself.  Interpretation of the barking
of dogs was another divinatory technique.  I recall a
catalog of the meaning of barking coming from different
directions... almost certainly offered by Kuno Meyer in
an early number of ZCP.