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.
Complete a level in co-op.
Get 100%. (Single Player Only)
Travel to Trollshaws as Bilbo.
Unlock all characters. (Single Player Only)
Craft every Mithril item. (Single Player Only)
Collect all the Mithril Bricks. (Single Player Only)
Collect more than 10,000,000,000 studs. (Single Player Only)
Complete 'The Mines of Moria' in under 15 minutes.
Defeat 42 Uruk-hai as Legolas.
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.
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)
Reach the top of Amon Hen without being caught once by Boromir. (Single Player Only)
Achieve True Adventurer in every level. (Single Player Only)
Fish perfectly 20 times by pressing the icon as it flashes.
Throw Gimli 30 times.
Complete 2 cooking puzzles.
Defeat Gollum as Gollum.
Unlock all the Map Stones in Middle-earth. (Single Player Only)
Walk into Mordor.
Complete the Bonus Level.
Complete 'Mount Doom'.
Complete 'The Black Gate'.
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 'Helm's Deep'.
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'.
Complete 'Amon Hen'.
Complete 'The Pass of Caradhras'.
Complete 'The Mines of Moria'.
Form The Fellowship of the Ring.
Complete 'Weathertop'.
Complete 'The Black Rider'.
Complete 'Prologue'.