You are confusing concepts here.
The dual citizenship remains in force regardless of what passport one uses to exit or enter. There may well be rules about what passport one has to use, but those rules do not effect the underlying citizenship.
Re the South African/Canadian. It sounds like you are relating a story second-hand. It appears that the current South African policy is to allow the entry with the foreign passport and issue a warning:
http://www.southafrica-newyork.net/h...ualcitinfo.htm
What is far more common is that countries will not let their dual citizens
leave the country with their foreign passport. I have heard a number of stories (second hand) of people who were born in Israel or Argentina, with virtually no connection to he countries, going in as a tourist on a foreign passport and being required to get an Israeli or Argentine passport to leave.
Originally Posted by
SOLTATIO
Many countries that allow dual citizenship and hence dual passports, require that in order for the dual citizenship to be approved and remain in force, the holder of the passport must enter his country of original citizenship (I'm assuming OP was a Canadian citizen before getting Irish citizenship), with the passport of the country of original citizenship and cannot enter with the passport of the "new"/"second" citizenship.
The example I have encountered was a South African obtaining Canadian citizenship later in his life and then trying to visit South Africa on his Canadian passport. He was refused entry as he held South African citizenship and needed to present his South African passport.
Cannot vouch for Canada or other countries, but I'd advise the OP to check with the local embassy.