Thanks for posting that interesting extract David. I'm having trouble with the notion that the Irish would simply call this star group 'seven stars', it's way too unimaginative and vague to be the name for one of the most impressive stars in the northern sky. And then having to create a hoop to jump through with the internal 'a', makes me extra nervous. If we look at what it is called in classical terms, its either the tail of the bear (despite the bear not having a tail) which Arkturus watches, or it is the plough of Bootes, or alternatively the oxen of the plough of Bootes.
So, I'm wondering if we could make sechtarré into seven-oxen-star ? that would at least make sense in star terms.
The term re which be any planetary body or star, or perhaps a star cluster (like we're dealing with here)? But given that we already have so relatively few instances of 're' anyway, I'm wondering if the 'ret' might be a (later?) term referring specifically to a star group?
This 'aret' is also uncomfortably close to the word for plough, so I wonder if some scribe a long time back also got a bit confused... I think it's a bit unlikely that this notion of ploughs and oxen was the ancient Celtic way of viewing these stars, so I think it's more likely to be someone trying to borrow a classical idea into Irish, which should mean that we are looking at either 'oxen' or 'plough' in some form or other.