O'Sullivan could easily have plumped for the more experienced Geordan Murphy to fill the void left by Horgan.
But in handing the Ulster man the number 14 jersey, the Irish coach has sent out a clear message - he is backing Trimble to rediscover the form which saw him tear Toulouse and South Africa apart earlier in the season.
Reflecting on his recall, after he was forced to sit out the wins over Australia and the Pacific Islanders, Trimble said: "I'm absolutely delighted, and maybe a little bit surprised, to have been picked and now it's really up to me to go out and do a good job.
"From an Ulster point of view, we've been very disappointed with our recent results and performance, but maybe this change of scene with the Six Nations is just what we need right now.
"I'm definitely going into Sunday's game with a positive perspective. I'm determined to do my bit for Eddie and repay this vote of confidence in me."
A try scorer against France on his Six Nations debut last February, Trimble is obviously keen to add to his tally of five tries in ten Test matches. There are few better places to score in world rugby than Cardiff's Millennium Stadium, a venue which the Ballymena clubman saw up close last May.
He did some media work at the ground for last season's Heineken Cup final, and insisted: "That trip gave me a chance to sample the atmosphere and I'm really looking forward to playing there. It should be a massive occasion - something special for the players and fans alike - and there's a lot of expectations on both teams to do well in this Six Nations."
With Ireland hotly-tipped to win the championship after three second-place finishes in the past four seasons, Trimble was staying tight-lipped about Ireland's chances, but admitted: "Things have gone well for us over the past year, including in the autumn, and I think that we've been steadily increasing our standard.
"We want to continually improve as a team and build upon what we've already achieved.
"For myself, things have maybe tailed off a little from when it was going so well in the autumn but the best players peak at the right time and I'm hoping that I'll respond again and step up to the mark."