I wish I had a definitive answer for you but all I know at this point is that
1) The 1914 Francis Guy's Cork Directory listed Mitchelstown with a Union and with registration districts Mitchelstown and Kildorerry, however we always know that by the time directories go to press there could have been changes made,
http://www.failteromhat.com/guy/mitchelstown.htm2) People have been mentioning a similar problem with Mitchelstown on genealogy boards at least since 2001 and it annoyingly nobody posts an informative followup.
http://archiver.rootsweb.ancestry.com/th/read/IRL-LIMERICK/2001-03/0983834881As far as I can tell it looks like Mitchelstown registration functions could have been absorbed by Tipperary, Clogheen, Fermoy, Mallow, and Kilmallock. There is probably also some absorption by Limerick.
Maybe an email to the GRO would help? Surely somebody there could give you a definitive answer. Do your existing certs you have say something like:
Registrar's District of Mitchelstown in the
Superintendent Registrar's District of Mitchelstown in the
Union of Mitchelstown?
If so, then when you contact them, make it clear that you want to know what happened to the
Registrar's District of Mitchelstown.
http://www.groireland.ie/contact_details.htmIn the meantime I am still poking around for a post-1915 list of the local Registrar's offices.