What I write here is extremely speculative: ogam is cognate with Gr. ὄγμος "furrow", Ved. ájma- "track", and it is likely that its original meaning was "furrow" although it is no longer used in that sense in Irish. achu apparently means nothing as such, but it could be aMiddle or Modern Irish mis-spelling for OIr. achad "pasture". Adding one and one, we get "until furrow and pasture are combined". In an agrarian society this is probably as absurd as having night and day together. Either you use a piece of land as a pasture or for crops, but you can't have your cake and eat it.
From: Old-Irish-L [mailto:[log in to unmask]] On Behalf Of Helen McKay
Sent: Freitag, 23. September 2016 06:14
To: [log in to unmask]
Subject: [OLD-IRISH-L] ogham agus achu
I have a question if I may, would anyone have any clues about what 'ogham agus achu' means?
Its from the story where Manannan is telling Angus how to trick Elcmar out of the Bru. There is another story that revolves around the idea of a day and night being all of time, and this is version seems to be along similar lines, where the end of time is referred to. (I've lifted this from mythicalireland website)
In the tale Altram Tighe Dá Mheadar (The Fosterage of the House of Two Vessels), Manannán advises Oengus on how to take Newgrange from Elcmar: "Command him not to come (again) to the house from which he departs until ogam and achu are mingled together, until heaven and earth are mingled together, and until the sun and moon are mingled together."
So the heaven and earth, sun and moon, are really typical elements found throughout Celtic lore, but what is the ogam and achu that seem to suddenly be part of the same conceptual space? And not only that, but so important that the idea leads it off. Presumably 'ogam' refers to the ogam alphabet, although eDIL says that in later usage it can also refer to the bardic written language in general. But what relevance does that have to the end of time? And eDIL only refers to this one instance of 'achu' with a question mark:
dil.ie/218 ?ACHU n fuagair do na tigeadh don tigh o teid no gu cumusgti ogham ┐ a.¤ re cheile, Ériu xi 190.32 .