Roos, a six-time Sunshine Tour winner, got the ball rolling with birdies at the 10th and 11th of Fota Island Resort, but drops at the 14th and 15th made him fade back into the middle of the pack.
His second nine was the front stretch of Fota Island and it suited the 32-year-old, who made a birdie at the first to put his bogey woes aside. Further gains at the fifth and ninth would give Roos a total of three-under-par 68 for the first round.
There is plenty at stake this week based on The Open Qualifying Series that forms a part of the tournament. The top three qualifiers not already exempt for The Open will earn the chance to play at the oldest major, and for Roos it could be a dream come true.
Justin Walters successfully played his way into The Open through sectional qualifying at the Joburg Open earlier this year, so his focus was on performing well in Ireland. He did just that on Thursday and shared fourth with his own 68.
Likewise Darren Fichardt, a steady veteran of both the Sunshine and European Tours. The four-time European Tour winner birdied three of his first five holes to set up a three-under-par total, which he retained until the end of the round.
Finland's Mikko Ilonen led the morning field with an impressive seven-under-par 64 and held a three shot cushion between himself and nearest rivals Edoardo Molinari and Matthew Baldwin.