"I'm frustrated, but at the same time I thought I saw some good signs out there", McIlroy said.
Also in the Scottish Open field is compatriot Henrik Stenson who next week goes to Royal Birkdale to defend his Open Championship title won 12 months ago at Troon, just down the road from Dundonald Links.
Playing alongside McIlroy, 2015 champion Rickie Fowler carded five birdies in a bogey-free 67.
"Anything that comes my way from here would be a bonus", said Harrington, who has played just nine events in 2017 and underwent surgery on a trapped nerve in his neck in March.
Rory McIlroy's frustrations with his game continued in the opening round of the Scottish Open as he posted a two-over-par 74.
Illonen's fortunes could not have been more contrasting, a run of five birides in six holes on his back nine helping the Finn move clear at the top of the leaderboard.
Ilonen is a five-time victor on the European Tour and has links pedigree, having won the British Amateur in 2000.
The Irishman defied the cold and rain later in the day to roll in his seventh birdie putt of the round at 17, although he was unable to take advantage of the par-five last as he remained two shots behind Ilonen, who arrived in Ayrshire on the back of four consecutive missed cuts.
Tournament favourite Rickie Fowler got off to a flyer with a round of 5-under 67 alongside a resurgent Ian Poulter, Callum Shinkwin and Andrew Dodt. On his third and fourth holes, he was in the middle of the fairway and less than 100 yards out, yet fell short and right with his approaches the latter into a burn to necessitate a drop for a double-bogey.
"But I remember a lot about that tournament, and maybe to this day it's the best golf I've ever played in my career".
It is the first time McIlroy is playing the event since 2014 when he shot a new course record 64 at Royal Aberdeen en route to sharing 14th place before then heading to Royal Liverpool and brilliantly claiming the Open Championship.
Fowler, who won at Gullane in 2015 but did not defend his title previous year due to scheduling difficulties, got up and running with a birdie at the 13th - his fourth - and added another at the next before breaking a run of six pars with three consecutive birdies from the third.
Of his seven birdies this week, five of them came on par fives.