The DIL's reference for entry one is "For pers. names of which gus forms the second element, see Ir. Gl. p. 69 n."
The text can be found here with the pertinent information:
http://www.archive.org/details/irishglossesmedi00stokuoft 342. Aengus (Oingus, Book of Armagh, 13, b. i, 19, a. i, 19, a. 2), gen. Oingusso, ib. 18 h. z, oingos, leg. Oingosso, ibid., a masc. u-stem, like Doilgus, gen. Doilgusso, Z. 18; Fergus, gen. Fergusso. Book of Armagh, 15, a. 2, fergosso, ib. 16 b. 2 (= W. Gwrwst?), Muirgus, Congus, Uarghus, and other nouns in -gus, = gustu ? as Dr. Siegfried suggests to me.
Note: Dr. Keeves has favoured me with a list of names in -gus, which he has collected from the Annals, Calendars, and Pedigrees. From this I select the following, in hopes that some may be identified with (Gaulish or Cymmric names: Alldghus, Artgus, Baothghus, Cuangus, Doedhghus, Donnghus or Dongus, Eachtgus, Faelgus, Fiangus, Fianngus, Flathgus, Lergus, Miodhgus, Nialgus, Saergus, Siiedgus. If Dr. Siegried's conjecture be established, we have here the Celtic representative of the Skr. r. jush, geuo, Lat. gustus, Eng. choose, Goth. kiusau. Cf. láimtech a des, diglach a gus, Seirgl. Cone. Atlantis ii. p. 382.
The Indo-european root suggested is geus, which is the root according to the American Heritage Dictionary, so gusto is related. Does anyone know how Fergus is declined?
(Message edited by seánw on August 06, 2009)