Ferguson carded a course-record 61 on Thursday at Galgorm Castle, one of two courses used for the event which sees men’s and women’s tournaments taking place simultaneously.
But the 26-year-old could only add a second round of 70 at Massereene Golf Club on Friday to remain nine under par, a shot ahead of Spain’s Borja Virto.
Italy’s Felipo Celli, who won the silver medal as leading amateur in the 150th Open at St Andrews last month, was a stroke further back on seven under, alongside Sweden’s Felix Palson.
“I’m still playing nicely but had a few more bad breaks today and missed a couple of the putts that I’ve kind of been making yesterday to keep my round going,” Ferguson said.
“(They were) good putts, but just kind of lipping out and yesterday they were kind of lipping in. You walk onto the next tee when they lip in, you feel amazing. When you walk on the next tee and they lip out you feel a bit flatter.”
The £2.5million purse will be split evenly in an event which was elevated to DP World Tour status following a successful inaugural edition on the Challenge Tour in 2019, when home favourite Stephanie Meadow won the women’s tournament and Jack Senior claimed the men’s title.
In the women’s event, American Amanda Doherty added a 67 at Massereene to her opening 67 at Galgorm to lead on 11 under par, one shot ahead of compatriot Lauren Coughlin and Denmark’s Emily Pedersen.
Ireland’s Leona Maguire and England’s Georgia Hall lie three shots off the pace, with former AIG Women’s Open champion Hall making a hole-in-one on the third at Massereene in her 69.
“We were slightly in between clubs but I kind of felt I couldn’t hit a nine iron and ended up hitting a hard wedge,” Hall said. “I couldn’t really see it, but I think it pitched a few yards short, spun, and then rolled in nicely.
“I turned to give my caddie the club and I thought it was a good shot and the spectators were clapping, and then they all like jumped up in the air and cheered. We knew it went in at that point.”