In older Gaelic the word for "brother" looked quite like its English and other cognates; "bráthair".

As in English, the word was used to denote and address a male sibling, and also to denote and address a monk or in its meaning as one's fellow man.

However, this dual use in a land known as "the island of saints and scholars" soon gave rise to some confusion, as monks were very numerous and very important and active in Irish life, and eventually there were many people who had brothers who were also monks, and monks who were siblings, I wonder if a bit like modern-day Tibet.

Therefore the word "bráthair" was no longer specific enough, and a new version arrived for actual siblings: "dearbh-bhráthair". "Dearbh" means "actual" or "real" or "blood_", as in "an actual real blood-brother", as opposed to the religious and community meaning used for addressing and naming monks and to a lesser extent scholars and others who serve the community, which was and still is "bráthair", and the word for a male sibling eventually universally became "dearbh-bhráthair", in use for the last few hundred years, which with the spelling reforms in the last century was given a new, shorter, simplified spelling more representive of its now somewhat squashed-up pronunciation; "deartháir".

So now, if you were speaking about a monk and a sibling in the same person, to say "Brother David is my brother", in currend Irish Gaelic you would say "Is deartháir liom an Bráthair Daithí".

Does that make any sense?

So. . .

Brother/brothers = Bráthair/bráithre (when used to address or mention monks or fellow men).

Brother/brothers = Deartháir/deartháireacha (when used for male siblings, but now also in some regions as a term of affection for a male friend, which to me shows it kind of morphing back into the original all-inclusive use of the original word, which started off the confusion and separation in the first instance).