There are a number of words for fields in Celtic languages. Some appear only or mainly in placenames. Here’s a selection:

| Old Irish (Goídelc) |
achad = expanse of ground; pasture, field; field of battle |
| Irish (Gaeilge) |
achadh [d̪ˠuːnˠ] = field (archaic, used mainly in placenames) |
| Scottish Gaelic (Gàidhlig) |
achadh [axəɣ] = field, plain, meadow; cornfield newly cut or ready for reaping |
Etymology: unknown
| Proto-Celtic |
*gortos = fence, enclosure, pen |
| Old Irish (Goídelc) |
gort = field, orchard, crop |
| Irish (Gaeilge) |
gort [ɡɔɾˠt̪ˠ] = (cultivated) field, orchard, (standing) crop |
| Scottish Gaelic (Gàidhlig) |
gort [gɔrˠʃd] = standing corn; enclosure; small field |
| Proto-Brythonic |
*gorθ = field |
| Welsh (Cymraeg) |
garth = field, close, enclosure, fold, pen, yard; fort |
| Cornish (Kernewek) |
gorth = field |
| Old Breton |
orth = field |
| Breton (Brezhoneg) |
garz = field |
Etymology: from the Proto-Indo-European *ǵʰortós (enclosure, hedge) [source], which is also the root of words yard and garden in English, via the Proto-Germanic *gardaz (enclosure, court, yard, garden) [source].
Words marked with a * are reconstructions.
| Irish (Gaeilge) |
machaire = plain; stretch of level ground, links, course; field |
| Scottish Gaelic (Gàidhlig) |
machair [maxɪrʲ] = extensive low-lying fertile plain, level country; extensive beach; ow and level part of a farm |
| Manx (Gaelg) |
magher = field, fertile land, campaign, chase, machar, sphere |
Etymology: unknown
| Proto-Celtic |
*rowesyā- = (field, open ground) |
| Old Irish (Goídelc) |
róe [r͈oːi̯] = battle-field, level piece of ground, fight, battle |
| Irish (Gaeilge) |
ré [rˠeː] = stretch of ground, level ground, field |
| Scottish Gaelic (Gàidhlig) |
raon [rˠɯːn] = field, (piece of) ground; plain; zone, area; field (of expertise); ambit |
| Manx (Gaelg) |
rheam = gamut, range, field, monarchy |
| Old Breton |
runt = mound |
| Breton (Brezhoneg) |
run = mound, hill |
Etymology: from the Proto-Indo-European *Hrew(H)os (open space, field). The English words rustic and rural come from the same root, via Latin [source].
| Proto-Celtic |
*kagyom = pen, enclosure |
| Gaulish |
cagiíun / *kagyom = enclosure |
| Old Irish (Goídelc) |
cai = field, orchard, crop |
| Proto-Brythonic |
*kaɨ = animal pen, enclosure, field |
| Middle Welsh (Kymraec) |
kay / kae = field, enclosure |
| Welsh (Cymraeg) |
cae [kaːɨ̯ / kai̯] = hedge, hedgerow, fence; field, enclosure; circle, sphere; barrier, obstruction |
| Cornish (Kernewek) |
ke = hedge, fence |
| Old Breton |
cai = hedge |
| Middle Breton |
quae = hedge |
| Breton (Brezhoneg) |
kae = hedge |
Etymology: from the Proto-Indo-European *kagʰyóm (enclosure, hedge) [source], which is also the root of word hedge in English, via the Proto-Germanic *hagjō (hedge) [source].
| Proto-Celtic |
*magos = plain, field |
| Gaulish |
*magos = field |
| Old Irish (Goídelc) |
mag [maɣ] = plain, field
ármag, árbach, ármach = field of slaughter, battlefield |
| Middle Irish (Gaoidhealg) |
magh = plain |
| Irish (Gaeilge) |
má [mˠɑː / mˠæː] = plain |
| Scottish Gaelic (Gàidhlig) |
magh [mɤɣ] = level country, plain
Magh Meala = Land of (Milk and) Honey (in mythology)
Magh Meall = elysium
magh na bàire = the plain of battle |
| Manx (Gaelg) |
magh = plain |
| Welsh (Cymraeg) |
maes [maːɨ̯s / mai̯s] = field, open country |
| Cornish (Kernewek) |
mes = open country |
| Old Breton |
maes = countryside, outside |
| Breton (Brezhoneg) |
maez = countryside, open field, outside, wide |
Etymology: possibly from the Proto-Indo-European *meǵh- (great) [source].
| Proto-Celtic |
*klowni = meadow |
| Old Irish (Goídelc) |
clúain = meadow |
| Irish (Gaeilge) |
cluain = meadow |
| Scottish Gaelic (Gàidhlig) |
cluain [kl̪uən̪ʲ] = green field, pasture, meadow |
| Old Welsh |
clun = meadow, moor |
| Middle Welsh (Kymraec) |
clun = meadow, moor |
| Welsh (Cymraeg) |
clun [klɨːn / kliːn] = meadow, moor; brake, brushwood |
Etymology: possibly from the Proto-Indo-European *klopni (wet).
Words marked with a * are reconstructions.
Sources: Wiktionary, Am Faclair Beag, Online Manx Dictionary, Teanglann.ie, eDIL – Electronic Dictionary of the Irish Language, In Dúil Bélrai English – Old Irish glossary, Geiriadur Prifysgol Cymru, Gerlyver Kernewek, Dictionaire Favereau, TermOfis, English – ProtoCeltic WordList (PDF), Etymological Dictionary Of Proto Celtic