In 2004 David and Dennis with input from some others looked at the 'song of Lugh', which is a speech that he makes before the Cath Magh Tuired.  Its a strange speech full of language that we can hardly recognise, little alone make sense of.  However, everything in it seems to be about the glory of battle, the great forces of the TDD, and an incitement to battle.  In the middle of this, is the sentence that seems similar to the saying of Manannan with 'ogham agus achu'.   This piece of Lug's speech is about battle, and not about time at all.  
 Incomairsid fri cech naie, go comair  Ogma sachu go comair nem & talom, go comair grioan & esqu
As best as I can do, the translation seems to be:
We should ask protection against every [naie] with the help of Ogma sachu, with the help of the sky and earth, with the help of the sun and moon.  

Because we are in the context of the Tuatha De Danann, there is reason to assume that Lug in his battle song is calling upon Ogma, who we know is The Champion of the TDD.   In this context, 'sachu' doesn't seem to be [AGU]s achu,  rather I'd think that Sachu is an epithet of Ogma, even if its not clear what the epithet means.  

Comparing this to the later version with Angus and Manannan:
Fuagair do na tigeadh don tigh o teid no gu cumusgti ogham 7 achu re cheile 7 no cu cumusgi neam 7 talman ar a
cheile 7 no cu cumusgi grian 7 esca ar a cheile.
'Warn him that he may not come to the house he leaves till [ogham agus accu]
be blent together, till heaven and earth, till sun and moon be blent together'.

The context is entirely different between the two, but there seems no doubt that the second one is 'modelled' on Lug's song.  So I think the question then is, has the author of the second piece misunderstood the original Ogma Sachu?  which wouldn't be at all surprising as no one then and now can read this ancient speech of Lug anyway.  Which then explains why 'ogham agus accu' means nothing in effect, because its a misinterpretation of something no longer understood.   Basically, its nonsense.  Well, that's the thought of the day anyway :-) 

Helen