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