If a person has a UK driving licence and is now back in Ireland what should he do if he incurs a speeding penalty. I assume he will get notification in the post.
The notice will be sent to the address of the registered owner of the car.
I don't believe they can apply points directly to UK licences yet, but they will keep a record, see here. Their insurer will need to be notified.
See this previous thread that suggests UK licences are only valid so long as the holder is resident in the UK. A fine of up to £1,000 applies if caught driving in the UK on a UK licence that doesn't list your current address.