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)
Complete 'The Mines of Moria' in under 15 minutes.
Complete a level in co-op.
Turn an enemy into a skeleton and make them dance.
Complete 'The Secret Stairs'.
Complete 'The Black Rider'.
Complete 'Prologue'.
Get 100%. (Single Player Only)
Collect all the Red Bricks. (Single Player Only)
Complete all the Fetch Quests in Middle-earth. (Single Player Only)
Unlock all characters. (Single Player Only)
Craft every Mithril item. (Single Player Only)
Collect all the Mithril Bricks. (Single Player Only)
Use the Ent Draught on Pippin.
Achieve True Adventurer in every level. (Single Player Only)
Travel to Isengard as every playable Hobbit.
Defeat Gollum as Gollum.
Dress a character completely in treasure items.
Unlock all the Map Stones in Middle-earth. (Single Player Only)
Travel to Trollshaws as Bilbo.
During 'Prologue', jump into the fires of Mount Doom as Isildur.
Throw Gimli 30 times.
Walk into Mordor.
Fish perfectly 20 times by pressing the icon as it flashes.
Equip Aragorn with the Mushroom Crown.
Craft the Fire and Ice Bows. (Single Player Only)
Complete the Bonus Level.
Complete 2 cooking puzzles.
Complete 'Mount Doom'.
Complete 'The Black Gate'.
Complete 'The Battle of Pelennor Fields'.
Defeat an Oliphaunt as Legolas.
Complete 'The Paths of the Dead'.
Complete 'Cirith Ungol'.
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 'Taming Gollum'.
Complete 'The Dead Marshes'.
Complete 'Track Hobbits'.
Complete 'Warg Attack'.
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 Pass of Caradhras'.
Form The Fellowship of the Ring.
Complete 'Weathertop'.