Dier, who set a course record with a 10-under-par 60 in Thursday's opening round, carded his second straight 67 with the help of a terrific drive and putt for eagle on the 18th to move to 16-under 194 for the tournament.
The 25-year-old Dier started the day with a three-stroke advantage, but made his first bogey of the week at the fourth hole and two more at holes 10 and 16. He had birdies at six, eight, 12 and 13 and was 1-under for the day with one hole to play.
Dier saved his best for last with a five-iron to the green and then converted a 20-foot putt for an eagle three.
"To finish eagle was good but I still felt like I left a few shots out there today," said Dier, who won his first tournament last season in the North West of Ireland Open. "But I'm happy with a 67."
Harrington birdied four of his last seven holes in his round of 64 to surge into a tie for second.
Roe carded a 66 with a birdie on 16 and also eagled 18 where his seven-iron shot landed three feet from the pin. "A 66 is always a good round here," said Roe, whose last win was in the Peugeot Open de France in 1994.
Lonard, who started the day three strokes behind Dier, shot a 68 and Fulke carded a 65 on Saturday.
Jamie Spence started the day tied with Lonard, but had a 69 and is tied with Raymond Russell at 11-under.
Defending champion Bernhard Langer shot a 69 for the second straight day and is 1-under for the tournament.