Not sure of all the intricacies of registering an English car but as Snipey says above a private individual has to register the car within 24 hours or risk seizure(I presume that if you were actually caught driving an English reg you would probably be OK for a few days,maybe a week)
I would suspect that a dealer would be allowed to drive on "garage plates" but I don't think he would do this indefinitely.
Open to correction but as far as I know it is illegal to drive on English plates in Ireland(same as it is illegal to drive on Irish plates in England)
Put it this way,if I was a car dealer who was importing English cars and taking the English plates off them immediately,I would not be impressed if a competitor in the same town was flouting the law by driving on English plates for months.
Even if it is not illegal it gives the impression of "pulling a fast one"