Here is the full list of all 48 LEGO The Lord of the Rings achievements. It takes around 30-35 hours to unlock all of the achievements on Windows.
During 'Prologue', jump into the fires of Mount Doom as Isildur.
Collect more than 10,000,000,000 studs. (Single Player Only)
Get 100%. (Single Player Only)
Unlock all characters. (Single Player Only)
Craft every Mithril item. (Single Player Only)
Collect all the Mithril Bricks. (Single Player Only)
Complete all the Fetch Quests in Middle-earth. (Single Player Only)
Collect all the Red Bricks. (Single Player Only)
Craft the Fire and Ice Bows. (Single Player Only)
Turn an enemy into a skeleton and make them dance.
Unlock all the Map Stones in Middle-earth. (Single Player Only)
Travel to Isengard as every playable Hobbit.
Fish perfectly 20 times by pressing the icon as it flashes.
Complete the Bonus Level.
Travel to Trollshaws as Bilbo.
Dress a character completely in treasure items.
Throw Gimli 30 times.
Defeat 42 Uruk-hai as Legolas.
Achieve True Adventurer in every level. (Single Player Only)
Complete 2 cooking puzzles.
Equip Aragorn with the Mushroom Crown.
Use the Ent Draught on Pippin.
Defeat Gollum as Gollum.
Complete 'Mount Doom'.
Complete 'The Black Gate'.
Walk into Mordor.
Complete 'Cirith Ungol'.
Complete 'The Secret Stairs'.
Complete 'The Battle of Pelennor Fields'.
Defeat an Oliphaunt as Legolas.
Complete 'The Paths of the Dead'.
Complete 'Osgiliath'.
Complete 'The Dead Marshes'.
Complete 'Helm's Deep'.
Finish one of Gimli's opponents as Legolas in 'Helm's Deep'.
Complete 'Warg Attack'.
Complete 'Track Hobbits'.
Complete 'Taming Gollum'.
Complete 'Amon Hen'.
Reach the top of Amon Hen without being caught once by Boromir. (Single Player Only)
Complete 'The Mines of Moria'.
Complete 'The Mines of Moria' in under 15 minutes.
Complete 'The Pass of Caradhras'.
Form The Fellowship of the Ring.
Complete 'Weathertop'.
Complete a level in co-op.
Complete 'The Black Rider'.
Complete 'Prologue'.