Grehan from Tullamore, a student at East Michigan University, had halves of 33 and 36 with birdies at the first, seventh, 11th and 14th.
Also on the 141 mark is Dublin’s Scottish Open Strokeplay champion Gavin Moynihan.
Moynihan, from The Island, added a 70 to his 71 of Wednesday. He carded one bogey — at the third — but birdied four, nine, 11 and 18.
Cormac Sharvin, the 2013 Irish champion from Ardglass, had a mixed round but stayed very much in contention with a 71 for 142.
Jack Hume of Naas shot a 73 yesterday for a halfway total of 143.
Dermot McElroy and Paul McBride just made the halfway cut with scores of 147. Jointly leading the field on 137 are the English pair of Ryan Evans and Ben Stow.