Tries from Harry Thacker, Javiah Pohe and Scott Wilson helped England to the win in Madrid, while fly half Billy Burns contributed seven points, and his replacement Sam Olver three.
The win rounds off a faultless season for John Fletcher's side, having already achieved wins against France, Scotland, Georgia and Wales.
England got off to a dream start and with two minutes on the clock were 7-0 up. A swift team move across field opened up space for hooker Harry Thacker to dive over, Burns converted from out wide.
Ireland had the chance to reduce the deficit with a penalty, but fly half Conor McKeon scuffed his kick wide of the posts.
With 15 on the clock England added their second try to go 12-0 up. Patient build up eventually freed Javiah Pohe who showed a nice step before going over the line - his third try in as many games.
But two quick penalties from Ireland's Mckeon brought the half to a close with England 12-6 up.
Burns added three points to England's lead early in the second period and prop Scott Wilson got England's third try fifteen minutes later - forcing the ball down after a successful driving maul to make it 22-6.
Ireland got their only try with ten minutes left to play when replacement Cian Kelleher sidestepped two English defenders to make it 22-13, but replacement fly half Olver gave England some breathing space and drew the scoring to a close with a penalty.
Speaking after the 25-13 victory, Fletcher said:
"We've gone unbeaten all season which in my eyes is a Grand Slam, we're really pleased with all the efforts, and it's not just this game - it's been throughout the campaign.
"A lot of these guys were in the Under 17 squad last season and they've come through and worked extremely hard and they deserve this win against Ireland. They deserve everything they get. Their effort, commitment and work rate was excellent and considering the nature of the game it was a really good performance."
Points: Thacker 1T, Pohe 1T, Wilson 1T; Burns 2C, 1P, Olver 1P..
England Under 18 starting XV
15 Zach Kibirige (Yarm School) (Newcastle Falcons)
14 George Catchpole (Leicester Grammar) (Leicester Tigers)
13 Tom Stephenson (Moulton College) (Northampton Saints)
12 Javiah Pohe (Wyggeston & Queen Elizabeth 1st) (Leicester Tigers)
11 Joshua Watkins (Bromsgrove School) (Worcester Warriors)
10 Billy Burns (Hartpury College) (Gloucester Rugby)
9 Callum Braley (Hartpury College) (Bristol Rugby)
1 Harry Rudkin (Wyggeston & Queen Elizabeth 1st) (Leicester Tigers)
2 Harry Thacker (Leicester Grammar) (Leicester Tigers)
3 Scott Wilson (Sedbergh School) (Newcastle Falcons)
4 Maro Itoje (Harrow School) (Saracens)
5 Charlie Ewels (Bryanston School) (Bath Rugby)
6 Ross Moriarty (Hartpury College) (Gloucester Rugby)
7 Gus Jones (c) (Eton College) (London Wasps)
8 Joel Conlon (Ivybridge Community College) (Exeter Chiefs)
16 Reece Marshall (Moulton College) (Northampton Saints)
17 Danny Hobbs-Awoyemi (Moulton College) (Northampton Saints) on for Harry Rudkin 38 mins
18 Alex Lundberg (Hampton) (London Wasps)
19 Jake Oldaker (Wellington College) (Bath Rugby) on for Charlie Ewels 57 mins
20 Josh Buggea (Hartpury College) (Worcester Warriors) on for Joel Conlon 47 mins
21 Tom Chapman (Colfe School) (Saracens) on for Callum Braley 55 mins
22 Sam Olver (Oundle Schhol) (Northampton Saints) on for Billy Burns 55 mins
23 Nick Carpenter (Filton College) (Bristol Rugby) on for Javiah Pohe 63 mins
24 Nathan Earle (Oaklands) (Saracens)
25 Max Northcote-Green (Millfield) (Bath Rugby)
26 Howard Packman (Moulton College) (Northampton Saints)
Content courtesy of RFU