If Ireland do win in Cardiff it would open the door for England by defeating the Scots at Twickenham in the final game of the championship.
It was the start of Gatland's first Grand Slam as Wales coach, a feat the Welsh repeated in 2012.
His side produced their finest.
"We haven't really been afforded that luxury so it's just a case of slotting people in but as I've said all through the Six Nations, it's been a really positive challenge for us to try to mix and match players and still try to get some cohesion".
"If you look at the stats England have made a hell of a lot more tackles than us in this tournament".
Peter O'Mahony, CJ Stander, Conor Murray and Keith Earls are the other Munster players in the starting 15.
"With that combination, I do think he is as ready as we can get him to be at this stage and I know he's incredibly motivated to be as ready as he can be".
England head coach Eddie Jones has suggested that Wales are starting to feel the pace
Wales have named an unchanged team for Saturday's Six Nations match against Ireland in Cardiff.
"We've worked in the down weeks and we've trained as hard as any team I've seen", said Gatland. It's about building belief and confidence in the players.
"If you want something bad enough and you really believe it can happen, then it often does".
"It will be the same approach we had with England", he said.
"When we've had success against Ireland in the past, we've tried to put pressure on Conor and Johnny and shut their space down".
Marmion - who was outstanding in the historic home victory over world champions New Zealand last November - has not featured in the previous four games due to injury.
His versatility in being able to play at flanker could make him a valuable addition to Schmidt's already hugely competitive World Cup squad. They're in a good spot and they've got a lot to prove in the last game, haven't they?'