The Roman Map of Britain Lougis Island of Lewis - Eilean Leodhais

Longis (r&c 299) for Lougis next
Limnou erhmoV  "Limnu (uninhabited)" (Ptolemy II 2 10) 

Ptolemy's islands Adru and Limnu are considered deserted because each is followed by erhmoV 'desolate, deserted, lonely'. Erimon (R&C 300) is the name recorded for Harris and Longis for Lougis Lewis, two names for regions of the same island. Adru and Limnu look like Greek adjectives adrou (adroV) 'great, thick, large' and limnou 'marshy' based on limnh 'marsh'.   In Johnston, "Many think corrup.of G. leoig, 'a marsh,' leogus, -ghuis, 'marshiness'; appropriate enough, but not agreeing with earliest forms." 

Adómnan records the island 'which in Latin may be called the Long Island Longa.'  I do not know Adómnan's source for the name, but this looks like a mis-read Louga. Lewis is recorded as Leodus, Lyoðhus ca. 1100.