Can you assume an approaching car will turn because its indicator is flashing in Northern Ireland?

No. The Highway Code warns that a vehicle indicator may have been left on accidentally, so a driver must judge whether it is safe before proceeding.

Rule 104 says watch other road users but do not rely on their signal alone. Check their road position and speed, whether they are actually slowing and whether there is enough time to pull out safely. Waiting for the car to begin turning is safer than treating the indicator as a guarantee.