Brian,

Bless me soul and saints preserve me, I'd forgotten about dear old St. 
Brigid.  She also is a good example of why this period is so problematic. 
How much of what we know about her is historical, how much borrowed from 
Celtic myths surrounding the goddess Brigid?

As for the Culdees or Ceile-De...

As with everything else, very little is known about them.  The name means 
Spouse of God, and they were Celtic anchorites who sought out the most 
inhospitable places of the British Isles to live.  There is disagreement 
about whether they were a development of Columban monasticism or something 
altogether different.  The similarities between them and Columban 
monasticism suggests to me that they were related.

The Culdees advocated strict solitude, seeking out remote caves and islands 
on which to live (Bardsey Island in S. Wales was a Culdee community).  They 
practiced a very rigorous round of prayers--based heavily on the recitation 
of psalms--and submitted themselves to a strict penitential system (at first 
employing "soul-friends" but later monastic seniors or abbots).  And they 
took learning seriously.  Later, when they formed monastic communities, they 
devoted themselves to creating the Irish illuminations like the Book of 
Kells.

Strangely, they were also known for being songsters.  Scholars believe that 
all of the penitential songs found in the Welsh Black Book of Carmarthen 
were originally Culdean.  At least in Wales, and likely too in their home 
base in Ireland, they seemed fond of composing songs of penitence, praise of 
God, and breastplate songs like the one traditionally ascribed to St. 
Patrick.  This has led to a theory that the Culdees were Christianized 
druidic bards, the Celtic equivalent of Greek philosopher-turned-Christian.

If I remember correctly, Rome never did recognize the Culdees as a 
legitimate order.  I have no idea what eventually became of them, though my 
guess is that the Benedictine movement finished them off as it did earlier 
forms of monasticism in France and England.

Mark+

PS  I don't normally carry this sort of stuff around in my head!  I'm 
currently teaching a 22-week adult Sunday school class on Christianity in 
the British Isles from 409-1066!!