Tonks moves from centre to full-back, while Hurrell resumes on the wing. Harrison is named among the England replacements.
England are defending the Six Nations title and have already beaten Italy (17-0) and Wales (28-15). Ireland are also unbeaten, topping France 9-6 and Italy 29-26 in the opening rounds.
"The boys have been very positive so far this week and the emphasis been on quality on the training ground, and being physically and mentally ready for a big challenge," said coach Mark Mapletoft.
"Ireland are a very competitive side at home and that's something I've seen for myself when we were beaten here two years ago with players of the quality of Danny Care, Jordan Turner-Hall, Matt Mullan and a number of other guys who are now playing in the Guinness Premiership in our side."
England Under-20s’ final two games in the Championship, against France on March 13 and Scotland a week later, will be shown live on Sky Sports.
England Under-20s (v Ireland, Athlone, February 27, 6.15pm)
15 Greig Tonks (Leicester Tigers)
14 Seb Stegmann (Harlequins)
13 George Lowe (Harlequins)
12 Tom Casson (Saracens)
11 Will Hurrell (Leicester Tigers)
10 Rory Clegg (Newcastle Falcons)
9 Dan White (Leeds Carnegie)
1 Ben Moon (Exeter Chiefs)
2 Jamie George (Saracens)
3 Shaun Knight (Gloucester Rugby)
4 James Gaskell (Sale Sharks)
5 Peter Elder (London Wasps)
6 Josh Ovens (Bath Rugby)
7 Calum Clark (Leeds Carnegie, captain)
8 Chris York (Harlequins)
Replacements
16 Arthur Ellis (London Wasps)
17 Bob Baker (London Wasps)
18 Carl Fearns (Sale Sharks)
19 Tom Sargeant (Harlequins)
20 Sam Harrison (Leicester Tigers)
21 Freddie Burns (Gloucester Rugby)
22 James Short (Saracens)
