Well IO, although gach
is not lenited, there are dialects that insert an s.
I ngach (in CO) = ins gach (in many Munster dialects)
Ó gach = ós gach
Do gach = dos gach
For an example of dos gach
where you can see that Tomás Ó Criomhthain used that in An tOileánach
I forget the origin of this s. I think it may be a development by analogy with in sna
, which then infects many preposition+article compounds and then is spread to gach
too. I'm sure someone has a better explanation - please post it!