Fòram na Gàidhlig

According to Scottish Gaelic in 12 Weeks (Chapter 9) it showcases the following structure using the Indirect Relative Pronoun:

* An sgioba leis an cluich mi - The team I will play with (lit. --with which I will play)
* An rathad mun an robh sinn a' bruidhinn - The road we were speaking about (lit. --about which we were speaking)

It then suggested an alternative using the Direct Relative Pronoun:

* An sgioba a chluicheas mi leis - The team I will play with (lit. --which I will play with it)
* An rathad a bha sinn a' bruidhinn mu a dheidhinn - The road we were speaking about (lit. --which we were speaking about it)

In asking around the common consensus appears to be that the indirect route is the way to go. Happy to do so. Now to the actual question!

Why does mun become mu dheidhinn in the direct format? Both mean "about", so is there a reason we just didn't use mun in both formats? As you can see, we use leis both times so it's not as if there is some rule against using the simple prep. pronoun at the end of the sentence. Is it a case of if you have a compound prep, use it as above, if not use the simple prep form both times?

It's not about the end of the sentence, as you say, which means it must be the mid-sentence position that's the special case.

In general, when we talk about a subject we either talk air (like giving a lecture on a topic in English) or mu dheidhinn the topic. (I can never remember whether that's the called a "compound preposition" or a "complex preposition" -- mu dheidhinn is one (preposition first) and còmhla ri is the other (preposition last).)

So in "An rathad mun an robh sinn a' bruidhinn" can be viewed as having lost the deidhinn/dheidhinn bit, rather than the other way round, which (as far as I can see) happens simply because deidhinn/dheidhinn expects a noun, but "an robh sinn a' bruidhinn" ain't a noun, so it sort of defaults back to a bare preposition.

Inntinneach. BTW are you the Niall who has been posting recently on the Learn SG FB Group (I post there under my real name - Pòl Cliaman )