Ms Deirdre Walsh:

It is an American variation, for the most part, and on some computer packages the title comes up spelled with a "c". The Act was written in 2005, so the title at the time would have been dietitian, as we are used to, and it is only with the passing of time that the use of the word spelled with a "c" has come on board in Ireland. If one were starting to write an Act now, one would probably use both titles, but back in 2005 the use of the word spelled with a "c" was not relevant.