Trimble praise for Henderson
Powerful ball-carrier Henderson will feature at both blindside flanker and second row against Italy on Saturday, with head coach Joe Schmidt admitting Ireland are "muddling through" working out his best position.
The 22-year-old will start at six in place of hamstring injury victim Peter O'Mahony for Ireland's penultimate RBS 6 Nations match.
Ireland wing Trimble tipped level-headed Henderson to cause havoc with ball in hand in his first Six Nations start.
"He's the man-child, he's got mutant strength for a 22-year-old, and he's still got to fill out his frame," said Trimble.
"He's going to be unbelievable and already at this stage he's pretty influential when he gets his hands on the ball.
"So he's going to make an impact on Saturday, definitely.
"Iain takes everything in his stride to be honest, I've never seem him unbelievably happy and I've never seem him really disappointed.
"He's just steady, you never really see a lot of emotion out of him, he just does the job.
"Going from training and the way that Joe (Schmidt) gets on with him, he's certainly someone that Joe likes a lot.
"It's very easy to appreciate all the things he brings to the game."
Craigavon-born Henderson has 34 Ulster appearances since his debut in April 2012, featuring 12 times for his province this term.
Schmidt has selected Leinster back-row duo Jordi Murphy and Tommy O'Donnell on Ireland's bench ahead of natural lock cover.
The former Leinster boss admitted Henderson is likely to move to lock in Saturday's latter stages, so Ireland can view him in both the back and second row.
"We're muddling our way through that to be honest," said Schmidt on defining Henderson's best position.
"We think he's athletic enough to play six, we think he's a fantastic line-out option to have there, and it's why we didn't select a second row on the bench as tempted as we were to include Mike McCarthy.
"We felt we had good options to go to, but we felt that Iain might finish the game in the second row for a period of time and that will give us a look at both positions.
"I'm not sure that Ulster have quite worked out where he fits either, because he plays a bit of both for them as well."