In the 25th edition of the event, Gallacher is in pole position to become the first man to successfully defend the title following a brilliant round of 63 on Saturday.
The 39-year-old Scot looked out of contention when he played his first eight holes at Emirates Golf Club in one over par to lie seven shots off the lead.
But the two-time European Tour winner then played the remaining 10 holes in 10 under, following a birdie on the ninth with seven more and an eagle in a brilliant back nine of 28.
Asked to explain the change in fortunes, Gallacher told Sky Sports: "I don't really know to be honest. It was quite a struggle to start with, especially the first three holes.
"I was not quite right but if you stay patient and keep focused it's amazing what this game throws up.
"I hit it stiff on nine and from then on just kept hitting it close, and if I hit it to 30ft I holed it. I was just trying to make as many birdies as I could and get a lead because I found the last time it's easier when you are leading than chasing.
"There's a lot of golf left to go but I will certainly be giving it my best shot."
Gallacher, who was denied a second win of 2013 in a three-man play-off for the Johnnie Walker Championship at Gleneagles, won by three shots here 12 months ago thanks to more amazing scoring.
A first round of 63 was followed by rounds of 70, 62 and 71, during which he made a total of five eagles, the last coming when he holed a wedge from 110 yards on the 16th in the final round.
He was also 10th in 2011 and second in 2012 and will not be overawed by playing in the final group with McIlroy either, having partnered the two-time major winner and Tiger Woods in the first two rounds.
McIlroy admitted he expected to win the title for a second time after holding a one-shot lead at halfway, but despite carding a 69, the 24-year-old was far from the brilliant form which saw him shoot an opening 63.
"It was okay," said McIlroy, who said he was not quite 100 per cent after a bout of food poisoning caused by takeaway food on Thursday night.
"I bogeyed the first again but birdied two and eagled three which was great.
"I didn't drive the ball particularly well, I'm probably going to work on a few things on the range and I felt like I hit a lot of good putts that just didn't go in.
"I felt like I left a few out there but it was a tricky day and shooting anything under 70 wasn't a bad score. Obviously Stephen had a phenomenal round, 63 today is a really impressive effort."
Told Gallacher was 10 under for the last 10 holes, McIlroy added: "I didn't know that. That's a great stretch of holes and hopefully he doesn't continue it tomorrow!
"I played with him the first two days and he played very well.
"Obviously he was on fire today, I just need to go out there and try to hit a few more fairways, focus on a few things in my game, not really focus on what he's doing and make a few putts and see where it puts me.
"It's the second week in a row for me going into the last day with a chance to win, it's a great position to be in."
McIlroy finished one shot behind Pablo Larrazabal in Abu Dhabi, having been penalised two shots for an incorrect drop in the third round.
American Brooks Koepka held the lead midway through the back nine until Gallacher's charge, the 23-year-old dropping shots at the 14th and 15th to card a 70 and finish 12 under alongside Denmark's Thorbjorn Olesen.
Olesen fired a flawless 65 but that was not the lowest round of the day, England's Steve Webster returning a rollercoaster 64 with nine birdies, an eagle and three bogeys to lie a shot behind on 11 under.
World number one Tiger Woods shot 70 to occupy a share of 37th place on five under.