Dier shot 66 Saturday to move to 16-under-par 200 at the Slieve Russell Golf and Country Club.
Through three rounds, the 24-year-old German is three shots better than playing partner Stephen Dodd, who shot 68 on Day 3. Five shots back at 11-under is Robert Jan Derksen (69), while Welshman Mark Pilkington is 10-under.
Playing in his first full season on the European Tour, Dier is searching for his inaugural win.
It appeared for a while on Saturday that a real shootout would be the call in the last round, but Dier ended all that speculation in the last few holes.
After Dodd ' a former British amateur champion ' tied the youngster at 13-under with a birdie at the par-5 13th, Dier shifted his game into overdrive, and birdied the 16th, 17th and 18th to pull ahead for good with a 66, which ties his best round of the season.
Click here to see Tobias Dier's scorecard.
It was not the best round of his career, however.
Dier shot a course record 65 at this very same venue last year, when he finished in a tie for fourth. That, too, was a career-best finish.
He is in great position to top that on Sunday.
It felt nice and easy, commented Dier following his round. This was my first experience of leading a European Tour event and I felt perfect. Hit one shot at a time and was in just the right frame of mind. Tomorrow we will see.
Full-field scores from the North West of Ireland Open