McIlroy jumped into a share of third place after making six birdies at The K Club, two of them coming at the 17th and 18th holes to thrill the deep galleries following their Northern Irishman on his homecoming on Friday.
"Amazing — the whole day," McIlroy enthused. "Even just walking to the range for the warm-up today, the support is absolutely incredible. I'm looking forward to playing in that atmosphere for the next couple of days as well."
The world No.2 may still find it hard work to reel in Lagergren, who led on 12-under par after 36 holes, and Adrien Saddier of France, who added a 66 to an opening 67 and was one stroke back.
The 33-year-old Lagergren has history at The K Club. Last year, when playing the Irish Challenge here on the adjoining Palmer South Course, he shot a course-record 63 and then holed out from a bunker to make eagle on the 72nd hole and earn a one-stroke victory.
"Ireland is treating me well," Lagergren smiled.
McIlroy was tied on seven under with Thorbjorn Olesen (71), Daniel Hillier (66) and Alfredo Garcia-Heredia (65).
The only survivor among the Australian challenge is Elvis Smylie, whose 69 would have been turned into something much more challenging if he had avoided calamity on the short 12th when his tee shot found the water and he recorded a triple-bogey six. He ended at two under, tied for 34th.
His two compatriots, David Micheluzzi, who shot a second straight 73, and Jason Scrivener, who was on three over, also after a 73, both failed to make the cut.
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