Probably best to find something in writing from the RTA as I suspect a phone call to 3 different people at the RTA on this issue would yield 3 different answers.
I rarely have the same type of tread tyres on the front and rear and have been pulled over for vehicle inspections on a number of occassions, they have never even questioned me about it (not to say it's right or wrong they may just lack observation skills?).
EDIT: just to clarify my post, I always have a matching set on the front and a matching set on the rear. As indicated below, different tyres from side to side is not good for obvious reasons.