If a republic of ireland car travelles on northern ireland roads?

Question

Do they get fined for speeding...and vice versa?

Answer 1

Yes and they can be taken to spend a night in the cells,

That will soon conme to an end when the penalty points system of both juristictions are brought into line,You will soon get penalty points for speeding in the north and vice versa

Answer 2

Yes, you can be fined for speeding if you are in fact speeding.  The issue may be enforceability of fine collection.