Irmo

(Lorien---the Vala, not the forest)

Umm. Gil-galad, the next word is supposd to start with the last letter of the word before. So I think the next word needs to begin with an "O".

woop my bad...

O huh...

Oathbreakers, the name given to the guys in the Pathes of the Dead

Sulimo, Manwe's surname.

Orald, one of Tom Bombadil's names.

Dark Elves

(The Avari)

And thanks, Gil-Galad, for being so good-natured.
(I tend to be too touchy :o ).

Stinker, Sam's other endearing nickname for Gollum

Renewer, The. Another name for Aragorn

Ringwraiths, another name for the Nazgûl.

Elessar Telcontar ,name with which Aragorn took over ruling Gondor

Ranugad, original form of Hamfast

Dark Lord

Dark Lord can be applied to both Morgoth and Sauron :D

Dunadan, (yet another) one of Aragorn's names.

Naugrim

The stunted people :D

Men-i-Naugrim

"Way of the Dwarves"

(a name for the Old Forest Road)

Mumakil, for the oliphaunts.

lathspell, Grima's term for Gandalf :D

Labadal, Sador's nickname given by Turin.

Gandalf "Stormcrow" name given by Theoden to Gandalf,

its "L" now ruts...

Lidless Eye, Sauron's scary name. ;)

Elbereth, Elves' name for Varda.

Hadhodrond

(Sindarin name for khazad-dum)

Dwarvish, another name for Khuzdul!

Haunted Mountain, another name for Dwimorberg

Ninquelote

(a name of Telperion)

Eluchil
 

A name given to Dior, meaning "Thingol's heir".

Little Folk
 

Take a wild guess what that means :D

Kud-Dukan

Rohirric name for the "Little Folk" :D

Neithan, the alias of Turin

Narquelie

Quenya name for the tenth month (in Sindarin Narbeleth)

Eregion, another name for Hollin

Nine Walkers aka the Fellowship

Serinde, Miriel's surname.

Elu, Sindarin for Elwe

Umbardacil, another name for King Telumehtar!

Lords of the West , another name for Valar

Teleporno, Celeborn's unfortunate Sindarin name ;).

Orodruin, or Mount Doom.

Necromancer, one of Sauron's names.