Defending champion Retief Goosen posted a 5-under-par 67 to stand alongside Frenchman Francois Delamontagne in second place. Jonathan Lomas and Gary Murphy are one stroke further back at minus-4.
Immelman opened on the back nine with five straight pars to begin his round. He drained a birdie putt on the 15th at The K Club to get into red figures.
The 25-year-old came back with a birdie on the 17th. Immelman made it two straight as he birdied the par-5 18th, his ninth. Around the turn, Immelman birdied the first to make it three in a row.
The three-time winner on the European Tour got to 5 under with a birdie on the par-5 fourth. Immelman stumbled to a bogey on the next however. He bounced back with a birdie on No. 6.
Immelman parred his next two holes before closing his round with a birdie on the par-4 ninth.
Goosen flew out of the gate with birdies on 10 and 11, his first two holes. He moved to 3 under with a birdie on the par-4 13th.
The two-time U.S. Open champion rolled home an eagle try on the par-5 18th to make the turn at minus-5. Goosen parred all nine holes on his second nine.
Northern Ireland's Darren Clarke posted a 3-under-par 69 in the opening round. He shares sixth place with Nick O'Hern, Brett Rumford, Roger Chapman, Jamie Donaldson, Gregory Havret and Graeme Storm.
Graeme McDowell of Northern Ireland opened with an even-par 72, that included a double-eagle on the par-5 fourth. Simon Wakefield is in a group one stroke further back at plus-one despite a hole-in-one on the par-3 eighth.
Newly crowned U.S. Open champion Michael Campbell struggled to a 2-over-par 74.