From BiblePortal Wikipedia

Fausset's Bible Dictionary [1]

Designation of two of David's guard, Ira and Gareb ( 2 Samuel 23:38;  1 Chronicles 11:40;  1 Chronicles 2:53). Jattir was in the mountains of Judah, one of David's "haunts," where he had "friends" to whom he sent of the Amalekite spoils ( 1 Samuel 30:26-27;  1 Samuel 30:31).

Holman Bible Dictionary [2]

 Exodus 4:18 1 Chronicles 2:53 Joshua 9:7 9:17 2 Samuel 23:38

Easton's Bible Dictionary [3]

 2 Samuel 23:38 1 Chronicles 11:40

Cyclopedia of Biblical, Theological and Ecclesiastical Literature [4]

or, rather, JETHERITE (Heb. Yithri', יַתְרַי , Sept. Ι᾿Εθρῖος and Ι᾿Εθερί , but Αἰθαλείμ in  1 Chronicles 2:53; Vulg. Jethrites and Jethrceus or Jethreus), the posterity of some JETHER mentioned as resident in Kirjath- jearim (A.V. "the Ithrites" [ 1 Chronicles 2:53]); probably the descendants of Hobab, the brother-in-law of Moses (who settled in this region,  Judges 1:16), and so called as being thus the posterity of Jethro the father-in-law of Moses. (See Kenite).

Two of David's famous warriors, Ira and Gareb, belonged to this clan ( 2 Samuel 23:38;  1 Chronicles 11:40). (See David). Ira has been supposed to be identical with; Ira the Jairite,"' David's priest ( 2 Samuel 20:26). According to another supposition, Jether may be only another form for Ithra ( 2 Samuel 17:25), the brother-in-law of David, and it is possible that the "Ithrites," as a family, sprang from him. According to still another supposition, the two Ithrite heroes of David's guard may have come from JATTIR, in the mountains of Judah, one of the places which were the "haunt" of David and his men in their freebooting wanderings, and where he had "friends" ( 1 Samuel 30:27; comp. 31).

International Standard Bible Encyclopedia [5]

ith´rı̄t ( יתרי , yithrı̄ , "excellence," "preëminence"): A family in Israel, whose home was Kiriath-jearim (  1 Chronicles 2:53 ). Among the 37 heroes of David, two are mentioned who belonged to this family, Ira and Gareb ( 2 Samuel 23:38;  1 Chronicles 11:40 ).