Thomas Bjorn, who answered the questions of whether his game would be affected by his collapse at the British Open with a 64 in the opening round, managed only a 2-over 74 on Friday. The Dane finished alongside Robert Karlsson of Sweden and Ireland's own Peter Lawrie at 6-under-par 138.
Lynn was five shots behind overnight leader Bjorn to start the second round at Portmarnock Golf Club and picked up three birdies over his first five holes to begin his move up the leaderboard.
The Englishman found trouble with a bogey at the eighth but recovered on the inward half. Lynn ran off back-to-back birdies starting at the 10th and added a birdie at the 13th to reach 8-under.
Lynn continued his fine play with a birdie at the 16th and followed that with a long birdie putt at the 17th to secure the 36-hole lead.
'I seem to get off the boil a bit in the middle of the season,' said Lynn, whose best finish this year was a tie for third at the Dubai Desert Classic. 'I just get a bit stale and bored. It can get a bit mundane and monotonous at times, although the enthusiasm comes back in the end. It's something that seems to kick in every year at some point.'
Lynn got a bit of a boost to his mid-season drag with a fine showing at the British Open last week.
'I thought the Open was a great experience,' said Lynn, who tied for 53rd at Royal St. George's. 'I loved it and I got a good buzz from playing in that tournament. I want to experience it more often and the only way to do that is get to the top of the leaderboard and play in the final groups at the weekend.'
Campbell, who posted his best first-round score of the season with a 66 on Thursday, opened with birdies at the second and the fifth before stumbling with a bogey at the sixth.
The New Zealander countered with a birdie at the 11th after his approach landed six feet from the cup. At the par-3 12th, Campbell knocked his tee shot inside 10 feet and drained the putt to make it two in a row.
'It's nice to be among the contenders again,' said Campbell, who posted a 69 in the second round. 'I've struggled the last three of four months. All I can do is control my own emotions and the golf ball, hit it, find it and see what happens.'
Peter Lonard carded a 4-under 68 to finish five shots off the lead at 5-under-par 139. Lonard was joined by fellow Australian Peter O'Malley and the English duo of Greg Owen and Gary Emerson in a tie for sixth.
Irishman Paul McGinley was one shot further back at 4-under-par 140. McGinley was joined by England's Ben Mason, Jean-Francois Lucquin of France and Sweden's Klas Eriksson in a tie for 10th.
The 36-hole cut fell at even-par 144 with defending champion Soren Hansen surviving to play the weekend on the number.
One prominent name who failed to qualify was local favorite Padraig Harrington, who had aspirations of becoming the first Irishman to win this event since John O'Leary accomplished the feat in 1982. Harrington played 36 holes at 1-over-par 143.