A brace from Oliver O'Neill and goals from Louie Barry and Andrew Moran either side of the break sealed the victory at Mounthawk Park ahead of the final game against Scotland on Friday.
Ireland were dominant from start to finish and deservedly went ahead in the 21st minute. The industrious O'Neill found the onrushing Barry with a lofted through-ball and after he expertly flicked the ball over the keeper, he finished to give the home side the lead.
Four minutes before the break, Ireland doubled their lead. O'Neill didn't give up on a long ball forward in difficult conditions and after he beat defender Keelan Williams to the ball, the Fulham forward fired home.
Paul Osam's side didn't let up after the break and grabbed their third nine minutes after the break. Substitute Sinclair Armstrong did well to find Bray Wanderers' Andrew Moran six yards out and he found the bottom corner.
With 15 minutes remaining, Ireland completed the scoring. West Bromwich Albion's Barry found O'Neill on the edge of the area and after he turned his defender, he slid the ball under the keeper for his second of the game.
Ireland could have scored more in the final minutes of the game as substitutes Armstrong and Oran Crowe went close but will settle for the 4-0 scoreline ahead of the final game of the tournament on Friday.
"We started the game very well, we needed to after the last 15 minutes of the game the other night," said U16 Head Coach Paul Osam.
"We did a lot of preparation coming into this game and it's great to see young players take instructions on board the way they did today.
"We had Wales penned back for long periods and had a lot of opportunities to score throughout the game. It was very important for us to keep a clean sheet as well so it's a real lift.
"It gives us an opportunity going into Friday's match so we'll go back and look at what we can improve on again going into that game."
Republic of Ireland: Daniel Rose (GK), Oisin Hand (C), CJ Egan-Reilly, Colin Conroy (Ben McCormack 66), Billy Vance (Sinclair Armstrong 41), Louie Barry (Lee Harkin 66), Andrew Moran (Kailin Barlow 56), Adam Wells, Oliver O'Neill (Oran Crowe 66), Jamie Doyle, Fionnan Coyle.
Wales: Luca Smith (GK), Keelan Williams (Harry Jones 41), Taylor Jones (Josh Edwards 51), Bryce Jones (Jay Williams 41), Matthew Baker, Ollie Ewing (C), Jadan Raymond (James Sweet-Lannin 51), Levent Gundogan (Ryan Viggers 63), Patrick Jones (Aaron Hillier 51), Cian Williams.
Referee: Eddie O'Mahony.