Isildur:

The elder son of Elendil, Isildur survived the War of the Last Alliance and cut the One Ring off of Sauron's hand, though instead of destroying it as he could have (being right on the slopes of Orodruin), he chose to keep it. However, the Ring betrayed him to his death, in the River Anduin, two years later. Isildur was technically the last of the High Kings of both Arnor and Gondor until the time of the King Elessar.

Ioreth:

One of the women of the Houses of Healing in Minas Tirith, Ioreth was able to recite snatches of rhymes for any herb, and was overly talkative.

Ilúvatar:

Ilúvatar was the one who created the Timeless Halls within the Void and the Ainur. He instructed them to sing, and from the result of their efforts created the Circles of the World which contained Arda. He also created those we call his children: the firstborn, the Eldar and later the Atani, or Secondborn. However, he also accepted the race created by Aulë, the Dwarves.

After that, he rarely interfered with Arda. The only time on record was at the end of the Second Age of the Sun when he removed the Undying Lands from within the Circles of the World and reshaped the lands into a sphere.

Ilúvatar is also called Eru.

Irmo

One of the Valar. He was more commonly known as Lórien, which was really the name of his residence. His name means 'master of desire' or 'desirer'. He is the brother of Mandos, or Námo, as his true name is. As his name says, Irmo was the master of visions and dreams. Irmo was married to Estë.

The Lord of the Rings
The Silmarillion
The Hobbit
The Complete Guide to Middle Earth by Robert Foster