Yes. If you are a British or Irish citizen, you do not need one. Similarly, if you have a visa or equivalent permission to live, work or study in the UK, or you are otherwise exempt from immigration control, then you do not need an ETA to travel to the UK.
Do these exceptions also apply to dual citizens? Yes, but you will need to prove your permission to travel by showing either a valid British or Irish passport, or any other valid passport that contains a certificate of entitlement.