Why the lenition of dachaigh after «a' dol»? Lenition seems very "rules based" but also expresses many subtleties of meaning. I read the following on Forum na Gàidhlig: "dhachaigh is a special case - it's dhachaigh with motion and dachaigh without motion". Is this the trigger here? Even native speakers don't always have coherent explanation for why lenition occurs. Occasionally it seems less "grammatical" than "stylistic".
dachaigh (home) is a noun that just gets lenited when used as an adverb (homewards)
Some words just typically get lenited even if there’s no apparent reason for it in the sentence. They are often listed already lenited in the base form in dictionaries (like Am Faclair Beag and Colin B.D. Mark’s The Gaelic-English Dictionary).
dhachaigh in the adverbial meaning of homewards is one of them, see dachaigh in Dwelly’s (marked as [Generally aspirated]) and dhachaigh (notice that the headword is already lenited) in Am Faclair Beag and in Colin Mark’s.
Taing. I just realized the "movement v. location" rule applies to «suas an staidhre» (movement) and «shuas an staidhre» (location) for upstairs and «sios/shios» for downstairs respectively (I think the "an" is optional). Cool! I'm happy to know all the ways lenition is semantically important & not just arbitrary aspiration ;-)
I am the first one to criticise anything we are given that is incorrect or causes confusion. But they cannot be expected to give a full grammatical and historical explanation of each word that is used. What they have said is that dhachaigh means 'home(ward)'. This is 100% true and 0% misleading, so it fine.
We just have to accept that some words are automatically lenited. Sometimes the reason is beyond an introductory course, and sometimes it is beyond everyone (thu). It is not the case that dachaigh has a different adverbial meaning. I have never heard it, and nor have Dwelly, AFB and Mark. It is used as a noun meaning a home, but it is not a very important word as people will often just use taigh, hence the common adverbial phrase aig an taigh 'at home'. You are probably better going with silmeth's clear and accurate explanation than with the apparently baseless claim made on the Forum.
Sidenote: people often misspell this word as *dachaidh. That might be how I discovered that Mark has done this twice (out of 78 examples). D