"Where is your brush?"
Translation:Cá bhfuil do scuab?
Why does "scuab" here not get a séimhiú? "s" can take a séimhiú, and "do" usually gives a séimhiú, for example "seomra -> do sheomra", so one would think it would be "scuab -> do shcuab".
For some reason "shcuab" sounds wrong to me, so I wasn't all that supprised that it was incorrect, but I don't know why that is. Anyone?
My rule of thumb is that it sounds wrong and is difficult to say, then it is wrong. Words starting with "sc-" don't take a séimhiú. The same goes for sp- and st-.
My modern (1990s) Irish grammar book fails to mention any exceptions to leniting S, but a 19th century grammar book (O’Donovan’s A Grammar of the Irish Language) mentions that S followed by B, C, D, G, M, P, or T is never lenited. I don’t know whether O’Donovan’s rule still applies in its entirety or not.
There aren't any words starting with SB, SD or SG in Ó Dónaill's Foclóir Gaeilge Béarla, so O'Donovan's rule might apply in theory, but not in practice.
After the spelling reform, sb-, sd-, and sg- were replaced with sp-, st-, and sc- respectively.
So technically, O’Donovan’s rule doesn't apply in its entirety, as only SC, SM, SP and ST are relevant in modern Irish.
Technically, it does apply in its entirety, since sb-, sd-, and sg- can still be relevant in modern Irish when discussing pre-reform topics, e.g. the book Mo Sgéal Féin.
A title in pre-modern Irish like "Mo Sgéal Féin" isn't really any more relevant than a title in French like "À la recherche du temps perdu". It's simply a quotation that follows the appropriate rules for the source. O'Donovan's rule applies for the same reason that French grammar and spelling rules apply to the title in French.
I would hesitate to say that the rule no longer applies in its entirety. Should you find yourself dealing with obsoleted forms, an obsoleted rule can become relevant again.
For example, I know people who still have difficulty with the reformed spelling. They might still use the SB, SD, and SG spellings, in which case the rule is still relevant to them.
In any case, I expect that scilling was specifically wondering about exceptions to O'Donovan's rule. I expect they already know this, but for the benefit of other readers of the discussion, S in SM is commonly (but optionally) lenited in Corca Dhuibhne. I'm not aware of other exceptions, but the Caighdeán notes two additional cases that probably didn't exist in O'Donovan's day: S is not lenited in SF or SV.