The Roman Map of Britain Maiona (Erimon) 7 Lougis Erimon Isles of Harris and Lewis, Outer Hebrides

Maiona Longis Eirimon, var, Erimon (R&C 298-300) next
Adrou erhmo
V "Adru (uninhabited)", an erroneous translation (Ptolemy II 2 10)
Limnou erhmoV  "Limnu (uninhabited)", an erroneous translation (Ptolemy II 2 10

    Ptolemy's islands Adru and Limnu have long been considered deserted because each is followed by erhmoV 'desolate, deserted, lonely'. Adru and Limnu, rather than the names of deserted islands, are simply the Greek adjectives adrou 'thick, stout, bulky' and limnou 'marshy'. Correspondingly recorded in the Cosmography are Maiona 'great'; Longis, for Lougis 'marshy'; and Erimon.   
   Harris is probably derived from Erimon, recorded as Heradh ca. 1500, Harrige 1542, the Harry 1549. 
Of the place-name Lewis, in Johnston, "Many think corrup.of G. leoig, 'a marsh,' leogus, -ghuis, 'marshiness'; appropriate enough, but not agreeing with earliest forms." Lewis is recorded as Leodus, Lyoðhus ca. 1100.

Eirimon (personal name, which sometimes indicates a place of origin)
The Corpus of Electronic Texts (CELT): Acallamh na Senórach I (Author: [unknown] 1100-1200 AD ) 193.38.2567 Herimon and H-Erimon (by far the more common forms)
The Corpus of Electronic Texts (CELT): The Irish version of the Historia Britonum of Nennius: Author: [unknown] 
The Corpus of Electronic Texts (CELT): Annals of Inisfallen, Pre-Patrician Section: Author [unknown]
and more.

    The equation of adrou and maiona needs some additional clarification. The idea that Harris might be individually considered large, or comparatively larger than Lewis should obviously be rejected. There is one unifying sense, that of fatness, luxury, richness of soil. It contrasts well with limnou and lougis 'marshy'. It has utility for other places of that name, and the sense survives in the modern Brittonic languages. 
The early names and the current agricultural situation of the islands, placed in a comparative context, would hold true only if limited to the two islands. Neither of the two are blessed with quantities of arable land. The recorded names are, of course, nearly two thousand years old, and the date of their naming is a matter of conjecture. Peat has been accumulating on the islands for several thousand years, and the islands have been inhabited to some degree since the Bronze Age. Harris has a significantly higher density of Iron Age sites than Lewis. So at their time of naming it would not be imprudent to consider that Harris was at one time considered the more luxuriant of the two. 

Perseus Project A Greek-English Lexicon
hadros thick, stout, bulky
        of animals, fine, fat
of fruit or corn, full-grown, ripe

Pokorny Indogermanisches Etymologisches Woerterbuch with further references to the Celtic languages (Pokorny is currently being revised.
Root/lemma: meg(h)- 'big'  (web p.123)
    Cf. OIr maissiu; W. Corn. mehin 'fat', MBret. bihin 'repletion' (*magesîno-); MW. maon (*majones) `the great ones', maith `long, great' (*maj-tio-), probably also OIr. do-for-maig 'L. auget', -magar `L augetur' [L. augeo- to increase, enlarge, enrich'], OW. di-guor-mechis 'added' (OW. ch from *-g-s-)

McBain's An Etymological Dictionary of the Gaelic Language
    mèith sappy, Irish méith, méath, Old Irish méth, Welsh mwydo, soften: *meito-; the e grade of the root seen in *moiti- (in maoth, q.v.), the root being mit, meit, moit ( meath, mèith, maoth).

Macfarlane's The School Gaelic Dictionary (Scottish Gaelic).
mèith a.fat, greasy, corpulent; rich, as soil

Kelly's Fockleyr Gaelg - Baarle (Manx Gaelic).
maa fat, greasy (as slug), luxuriant
    mea rich, fat, thick, luxuriant
    meaghey fatten
    meeaynlyssagh fatty

    There several other related entries in the British section:
    1) a town, Masona (R&C 21) for Maiona
    2) a town, Maio (R&C 120) for Ma(g)ione
    3) a town, Morionio (R&C 30) for Maionio
    4) a river, Maina (R&C 238) for Maiona

    I may be wrong equating adrou with maiona. I've overlooked the preceding entry Gradena, var. Grandena in two manuscripts. PNRB equates the name with L. grandina 'hail', but a name resembling L. grandis 'full-grown, great, large' - Grandea ? - could be the source of Ptolemy's translation adrou.

    At the end of this section dealing with the islands, the cosmographer states that he would have liked to have named the individual islands of the Orkneys, but the names are different. This should suggest two differing sources for this area. Were there to be a second map of the area, is the idea supported by the phrase Item ad aliam partem dicitur insula?