England made a strong start and had the first points on the board with two minutes on the clock after a long-range Mallinder penalty.
Ireland hit back soon after and had the first try ten minutes later, quick footwork opened up space for centre Jack Power to score between the posts.
England came back though, and had their first score through Prell who spotted a gap in the Irish defence and backed his pace to dart through, Mallinder added the extras to make it 10-7 with 23 played.
In what proved to be a topsy-turvy first half, Ireland had their second try minutes later, rumbling over from close range.
An intercept try from Slowik gave England back the lead, and John Fletcher's side never look back.
In a dominant second half England extended the lead through Prell after a good break from Mallinder before Bainbridge rounded the win off, touching down at the bottom of a rolling maul.
Head Coach Fletcher said:
"We're pleased to get the win, and to be honest I did think that we were the better side, particularly in the second half - but huge credit to Ireland, they're a very good side and made it extremely difficult.
"In terms of our performance, I think that we were much better in the second half, and it's a credit to our lads - they're a tight knit group, and it bodes well for the future.
"We've had a great tour here in Poland, they want the game to grow and this tournament will have helped that. From the hotel to the stadiums, we've enjoyed every minute, which is what it's all about."
Captain Sam Underhill added:
"It was an extremely tough game, and we're just pleased to come out of it with a win. Ireland came at us from the first minute but I think that we were patient and scored at important times, which is what made the difference. As a group we're really close and while I was fortunate enough to lift the trophy it was a real team effort."
Leeds full back Taylor Prell said:
"I'm pleased to get two tries, but I'm more pleased to have got the win. Massive credit to Ireland, it was a great game to play in, and both sides played their part. We've grown close as a squad and have had three good wins - Poland has been great, and we've enjoyed every minute."
Points: Prell 2T, Slowik 1T, Bainbridge 1T; Mallinder 2P, 2C
England Under 18 starting XV against Ireland
15 Taylor Prell (Giggleswick School, Leeds Carnegie)
14 George Perkins (Oaklands, Saracens)
13 Lucas Slowik (Filton College, Bristol Rugby)
12 Charlie Thacker (Leicester Grammar, Leicester Tigers)
11 Kai Little (Hartpury College, Exeter Chiefs)
10 Harry Mallinder (Rugby School, Northampton Saints)
9 Jack Maunder (Blundells School, Exeter Chiefs)
1 Lewis Boyce (Prince Henry's Grammar, Leeds Carnegie)
2 Ollie Adams (St Pauls Catholic College, London Irish)
3 Josh Ibuanokpe (Dulwich College, Harlequins)
4 Stanley South (Whitgift School, Harlequins)
5 George Nott (Kings School Chester, Sale Sharks)
6 Joe Batley (Hartpury College, Gloucester Rugby)
7 Sam Underhill (Sir Thomas Rich's School, Gloucester Rugby, captain)
8 Will Safe (Hartpury College, Gloucester Rugby)
16 James Fish (Trent College, Northampton Saints) on for Ollie Adams 58 mins
17 Owen Hills (Queen Elizabeth Wyggeston College, Leicester Tigers) on for Lewis Boyce 50 mins
18 Jack Stanley (Truro College, Exeter Chiefs) on for Josh Ibuanokpe 55 mins
19 Callum Chick (Gosforth Academy, Newcastle Falcons) on for Sam Underhill 55 mins
20 Huw Taylor (Old Swinford Hospital, Worcester Warriors)
21 Josh Bainbridge (Prince Henry's Grammar, Leeds Carnegie) on for Stanley South 41 mins
22 Alex Mitchell (Lymm High School, Sale Sharks) on for Jack Maunder 50 mins
23 Theo Brophy Clews (Abingdon School, London Irish)
24 Joe Marchant (Peter Symonds, Harlequins) on for Lucas Slowik 35 mins
25 Rotimi Segun (Stowe School, Northampton Saints) on for Kai Little 35 mins
26 Cameron Cowell (St Pauls Catholic College, London Irish)
Content courtesy of RFU.
Images courtesy of Greg Underhill