Defending champion Angel Cabrera bogeyed the 18th to fall one shot off the lead at 9-under-par 207.
Harrington began the day with a two-shot edge over Cabrera and picked up his first birdie at the par-5 third. He bogeyed the fourth but got that shot back with a birdie at the par-4 sixth after his approach landed six feet from the cup.
Cabrera meanwhile drained a 12-foot putt at the sixth for his third consecutive birdie to stand alongside Harrington at minus-10. Harrington found trouble off the tee at the seventh en route to a bogey to give Cabrera sole possession of the lead for a brief moment.
Harrington hit his second shot to nine feet for a birdie at the 10th to regain a share of the lead. At the par-5 15th, Harrington chipped his third shot from off the green to four feet for a birdie and the outright lead at 11- under.
The 31-year-old closed his round with a blunder. He found a greenside bunker at the last on his way to a bogey that put him side by side with a few players who had much better rounds on Saturday.
Scahill started with two birdies over his first three holes and drained a putt from the fringe at the fourth to move to 6-under. He added a birdie at the eighth and moved to minus-9 with a pair of birdies from the 11th.
At the par-4 16th, Scahill dropped his second shot within a foot of the cup for a birdie to match the Brabazon Course record with a 65.
'It's a fantastic position to be in,' said Scahill, who is seeking his first title on the European Tour. 'I managed to get some momentum going on early in the round and kept it going. I had to make a couple of great up and downs and I feel good about my position going into the last day.'
Casey picked up two birdies and a bogey on the outward half to make the turn at minus-5. The 25-year-old caught fire on the back side with five birdies for a round of 66.
'I am very happy with that,' said Casey. 'My course management was pretty good and on cold days like this it helps to be able to reach holes like the 17th with a 3-wood and 5-iron. It's a huge advantage. I would have settled for being one off the lead going into the last day but I am pleased to be sharing the lead.'
Rolf Muntz finished four shots off the lead at 6-under-par 210. He was followed by David Dixon and Ricardo Gonzalez at 4-under-par 212.
Paul Lawrie posted a 70 to finish one shot further back at 3-under-par 213. The 1999 British Open champion was joined by Brian Davis, Simon Khan and Jose Manuel Lara in a tie for eighth.
Colin Montgomerie collected two birdies and two bogeys for a round of even-par 72. He finished alongside Niclas Fasth and Bradley Dredge at 2-under-par 214.