The 22-year-old will start at tighthead prop against the Grand Slam-seeking Irish - and admits the position is new to him.
"I've had a couple of games as a replacement on the tighthead - it's a different feeling to starting a game," said Jenkins, who replaces Swansea' Ben Evans and also gets the nod ahead of Llanelli ball-carrier Martyn Madden.
"I haven't played many games there, but I do feel comfortable. I had two scrums against England and I didn't get a feel for it. There were more scrums against Scotland and it was difficult to start with.
"I'm learning with every game although I guess the only way you're really going to learn is by playing.
"Wales Under-21 coach Chris Davey taught me a lot about scrummaging and I learned a lot from Mike Griffiths, the old Wales and Lions prop when he was helping out at Ponty. Other than that, I've played two friendly games for Pontypridd in the position last season. My clubmate Duncan Bell has given me a few tips of his own, although I'm not convinced they're all going to work.
"Ireland have got a strong scrum and lineout and it's going to be a tough game. They do the basics well, but we have got to be aggressive and stand up to them.
"I like to get around the park and if there's a tackle to be made then I look to put a big tackle in." "
At 18st 10lb and 6ft 2in, Jenkins is seen as the cornerstone of the Welsh pack for the next decade, and he's ready for the challenge.
"The tighthead role is certainly different from loosehead - it's a lot tougher physically," Jenkins said.
Veteran Wales captain Jonathan Humphreys says his young frontrow colleague has what it takes to succeed at Six Nations level.
"I've seen Gethin train and I've seen him play. He works hard and he's got a big heart," Humphreys said.
Wales coach Steve Hansen has called on his young Wales side to make their mark in history by breaking the 20-year Irish jinx in Cardiff.
Wales have not beaten Ireland on home soil since their 16-9 success at the former National Stadium way back in 1983.
So the Wales coach upped the stakes ahead of the Six Nations clash by urging his players to end the miserable home run against their Celtic rivals.
"Records are there to be broken," said Hansen. "That run has been brought up this week and it is a great opportunity for this team to create a piece of its own history.
"Perhaps the record is a reflection on how things have changed over time.
"Ireland have gone through the same troubles as us and were criticised for being a poor team.
"But they stuck together, dug in and now they can compete with the best teams in the world - that is what we need to do right now."
Wales: R Williams (Cardiff); M Jones (Llanelli), M Taylor (Swansea), T Shanklin (Saracens), Gareth Thomas (Bridgend); S Jones (Llanelli), G Cooper (Bath); I Thomas (Llanelli), J Humphreys (Bath, capt), G Jenkins(Pontypridd), R Sidoli (Pontypridd), G Llewellyn (Neath), C Charvis(Swansea), M Williams (Cardiff), D Jones (Llanelli).
Replacements: M Davies (Pontypridd), M Madden (Llanelli), S Williams (Northampton), Gavin Thomas (Bath), M Watkins (Llanelli), I Harris(Cardiff), D Peel (Llanelli).
Ireland: G Murphy (Leicester); J Bishop (London Irish), B O'Driscoll (Blackrock College/Leinster, capt), K Maggs (Bath), D Hickie (St Mary's College/Leinster); D Humphreys(Dungannon/Ulster), P Stringer (Shannon/Munster); M Horan (Shannon/Munster), S Byrne (Blackrock College/Leinster), J Hayes (Shannon/Munster), L Cullen (Blackrock College/Leinster), M O'Kelly (St Mary's College/Leinster), A Quinlan (Shannon/Munster), K Gleeson (St Mary's College/Leinster), A Foley (Shannon/Munster).
Replacements: F Sheahan (Cork Constitution/Munster), J Fitzpatrick (Dungannon/Ulster), D O'Callaghan (Cork Constitution/Munster), E Miller (Terenure College/Leinster), G Easterby (Llanelli), R O'Gara (Cork Constitution/Munster), J Kelly (Cork Constitution/Munster).