Apparently this phrase means "in uproar".

It is a little awkward to derive.

Ó Dónaill's dictionary (NÓD1977) has this as derived from bruíon meaning "hostel, fairy palace" and not bruíon meaning "strife". It might seem those two words are the same, but the first was written bruidhean in the old script, and the second bruighean. Then Ó Dónaill's links it to caorthann "rowan tree". The fairy palace of the rowan tree. What about caorthainn "blaze, rage"? Why not the "strife of rage" instead of the "fairy palace of the rowan tree"? I don't think Dinneen has this...

The passage is:

D’éirigh eatarthu féachaint ceoca buíon a bheadh ar tosach. Bhí fearg ag teacht orthu. Ní ghéillfeadh aon bhuíon acu do bhuíon eile. Shíleas go mbeadh sé ’na bhruín chaorthainn eatarthu.

There is a famous story in the Fenian Cycle:

Cath Bruíon Caorthainn

I wonder if there is some history behind this phrase. The only example I can find is at http://www.ucc.ie/celt/published/G402571/text001.html , and that poem seems to relate to some old myths.

Maybe there was a rowan tree fairy palace in Irish myths. Or maybe rowan-tree fairy palace became confused with raging strife, owing to the similarity of the phrases, and then the one was used for the other?

Here is Lady Gregory's version of the Tale.

http://www.sacred-texts.com/neu/celt/gafm/gafm69.htm

Thank you!

I was looking for an Irish Verison, but I can't find one online.

However, you will find the names of Fenian Tales being used in this way: another one is "Cath na bPunann"

Aonghus, would you agree with the following entry for my foclóirín on this phrase?

Bruíon chaorthainn: ’na mbruín chaorthainn, “in uproar”. This phrase is a reference to the tale Cath Bruíon Chaorthainn in the Fenian cycle of myths, which relates how the Fianna were tricked into an enchanted house of rowan trees (bruíon chaorthainn) where they were held by a spell until their captors were killed and the spell broken.

Sounds accurate.