The man was also being questioned over the abduction of a 17-year-old girl, said a Greater Manchester police spokesman.
Ms McGrath, 27, was stabbed to death as she arrived to collect her boyfriend from the pub in Bramhall, Stockport. Her father Brian McGrath, 55, is a district judge in the family division.
A man was arrested early yesterday in Caernarfon, North Wales, in connection with the kidnapping of the 17-year-old girl in Stockport. It is understood she was abducted as she was driving to work on Saturday morning. She later escaped from her kidnapper's car.