Words for king and related things in Celtic languages.

Brian Boru, High King of Ireland (1002-1014) / Brian Bóruma mac Cennétig, Ard-Rí ar Éirinn

Proto-Celtic	*rīxs/*rig- = king
Celtiberian	reikis = king
Gaulish	rīx = king – appears in names like Katurīx, Suādurīx, *Werkingetorīx (Vercingetorix) and Asterix
Primitive Irish	ᚏᚔᚌᚐᚄ (rigas) = (of the) king
Old Irish (Goídelc)	rí [r͈ʲiː] = king rígdae = regal, kingly rígdún = palace
Middle Irish (Gaoidhealg)	rí, ríg, rig = king, head, chief rígán = a sub-king, chief rígamail, rioghamhail = royal rígdacht = kingliness, kingly qualities, a kingdom rígda(e) = a royal dwelling place, palace rígrach = = royal, kingly
Irish (Gaeilge)	rí [ɾˠiː] = king, sovereign, monarch ríoga = regal, royal ríora = kings, royal persons, royalty, dynasty
Scottish Gaelic (Gàidhlig)	rìgh [r̪ʲiː] = king banrìgh = queen àrd-rìgh = high king mòr-rìgh = emperor rìoghachadh = reigning, reign rìoghachd = kingdom, realm
Manx (Gaelg)	ree = king reejerey = cavalier, chevalier, king, knight, lord, paladin, prince reeoil = kinglike, monarchy, regal, royal reeaght, reeriaght, riaght, ream = kingdom
Proto-Brythonic	*riɣ [ˈriːɣ] = king
Old Welsh	rig = king
Middle Welsh (Kymraec)	ri, rhi = king rhiaidd, rhïaidd = regal, royal, noble, generous rieu, riav = king, lord, ruler, chieftain rriawdr, riawdr = lord, ruler, chieftain, God
Welsh (Cymraeg)	rhi [r̥iː] = king, prince, lord, ruler, chieftain rhiaidd = regal, noble, aristocratic, dignified rhiau = king, lord, ruler, chieftain rhiawdr = lord, ruler, chieftain, God
Middle Cornish (Cernewec)	ruy, ruif = king, sovereign, ruler ryal, real = royal, kingly
Cornish (Kernewek)	riel = regal, royal
Old Breton	ri = king
Middle Breton (Brezonec)	ri, rí = king

Etymology: from Proto-Indo-European *h₃rḗǵs (king, ruler) [source], which is also the root of the German word Reich (empire, realm), and the English suffix -ric, as in bishopric (a diocese or region of a church which a bishop governs) [source].

Proto-Celtic	*rīganī= queen
Gaulish	rigani = queen
Old Irish (Goídelc)	rígan = queen
Middle Irish (Gaoidhealg)	rígain, rígan, rígnae = queen, noble lady
Irish (Gaeilge)	ríon = queen, queenly, noble, lady, fair maiden banríon = queen (ban)ríonacht = queenship ríonaí = queenly ríonaigh = to queen (chess)
Scottish Gaelic (Gàidhlig)	rìghinn [r̪ʲi.ɪn̪ʲ] = princess (archaic) rìbhinn [r̪ʲiːvɪn̪ʲ] = maid, maiden, girl, Venus, adder, queen (archaic) banrìgh = queen
Manx (Gaelg)	reeven = queen rein = queen ben-rein = queen
Proto-Brythonic	*rriɣėn [r͈iˈɣe̝ːn] = queen
Middle Welsh (Kymraec)	riein, riain = (young) woman, girl, damsel, lass, maiden, virgin, queen, noble girl, lady
Welsh (Cymraeg)	rhiain = (young) woman, girl, damsel, lass, maiden, virgin, queen, noble girl, lady

Etymology: from Proto-Indo-European *h₃rḗǵnih₂ (queen), from *h₃rḗǵs (king, ruler) [source]. Words from the same roots include reine (queen) in French, reina (queen) in Spanish, erregina (queen) in Basque, and regină (queen) in Romanian [source].

Middle Welsh (Kymraec)	rhỼyf, rwyf, rỼy = king, lord, ruler, chieftain, leader rwyfaỼ, rwyfaỼ = to have dominion over, govern, rule rwyuannus, rhwyvanus = royal, kinglike, governing ruyvenit = prince, king, ruler rhwyfiadur, ruyfadur, rwyfyadur, rỼyfyadur = chief(tain), lord, leader, ruler, protector
Welsh (Cymraeg)	rhwy(f) = king, lord, ruler, chieftain, leader rhwyfo = to have dominion over, govern, rule, lead, be ostentatious, live in splendour rhwyfanes = queen, lady rhwyfaniad = reign, government, rule, kingdom rhwyfanus = royal, kinglike, governing rhwyfenydd = prince, king, ruler rhwyf(i)adur = chief(tain), lord, leader, ruler, protector
Old Cornish	ruifanes = queen ruifadur = chief(tain), lord, leader
Middle Cornish (Cernewec)	ruy, ruif = king, sovereign, ruler ruifanes, ruivanes = a female ruler, queen
Cornish (Kernewek)	ruvanes = queen ruvaneth = kingdom
Old Breton	roe = king
Middle Breton (Brezonec)	roe, roé, roue, rouè = king rouanes, roanez, rouanés = queen rouantelez, roeantelez = kingdom roueel = royal roueeler = royalist roueelezh = royalty roueelouriezh = royalism roueoni = reign
Breton (Brezhoneg)	roue = king rouanez = queen rouantelezh = kingdom, monarchy roueel = royal roueelezh = royalty roueelouriezh = royalism

Etymology: from Old French roi (king), from Latin rēgem (king, ruler, despot, tyrant), from Proto-Italic *rēks (king), from Proto-Indo-European *h₃rḗǵs (king, ruler) [source].

Words from the same Latin root include re (king) in Italian, roi (king) in French, viceroy in English and rey (king) in Spanish [source].

Proto-Celtic	*brigantīnos = (someone) pre-eminent, outstanding
Proto-Brythonic	*brɨɣėntin [brɨɣe̝nˈtiːn] = lord, king
Middle Welsh (Kymraec)	breenhin, breennin, breyenhin, brenhin = king brenhynes, brenhines = queen, goddess, queen bee brenineiziaw = to become kingly brenhynyaeth, brenhinyaeth = royalty, kingship, reign
Welsh (Cymraeg)	brenin [ˈbrɛnɪn / ˈbreːnɪn] = king, sovereign, monarch, head of region brenineiddio = to make/become regal/kingly, make (sb) a king breninesaidd = queen-like, queenly breninesol = queen-like, queenly, pertaining to a queen breningarol = royalist breniniaethu = to reign, govern breninoldeb = kingship, royalty brenhines = queen, goddess, queen bee brenhiniaeth = royalty, kingship, reign, government
Middle Cornish (Cernewec)	brentyn, bryntyn = privileged, sovereign, noble, excellent
Cornish (KerneweK)	bryntin, brentin = grand, great, noble, splendid, superb
Old Breton	brientin, brientinion = king
Middle Breton (Brezonec)	brientin = aristocrat brientinelezh = aristocracy
Breton (Brezhoneg)	brientin = aristocrat brientinel = aristocratic brientinelezh = aristocracy brienteg, brientek = privileged brientekaat = to favour briental = royalties

Etymology: from Proto-Indo-European *bʰérǵʰonts (high), from *bʰerǵʰ- (to rise up, ascend) [source].

From the same Proto-Celtic root, , we get the tribal name *Brigantī (Brigantes – a Celtic tribe that occupied much of what is now northern England), Brigantia (a Celtic goddess of victory, and the territory of the Brigantes), the name Bridget, the Irish name Bríd, the Scottish Gaelic name Brìghde, the Manx name Breeshey, and the placenames such as Bragança (a city in northeastern Portugal), Bregenz (a city in western Austria) and Brianza (a city in northern Italy), [source].

English words from the same PIE root include bourgeois, burrow, fort and maybe burgler [source].

Middle Welsh (Kymraec)	mechdeyrn, mychdeyrn, machdeyrn = king, lord, monarch, emperor mechteyrnged, mychteyrnged = tribute due to an overlord from an inferior lord
Welsh (Cymraeg)	mechdeyrn, mychdeyrn, machdeyrn = king, lord, monarch, emperor, God, tributary prince, viceroy mechdeyrnged, mychdeyrnged = tribute due to an overlord from an inferior lord
Middle Cornish (Cernewec)	mychtern = sovereign, king mychternes = queen mychterneth = sovereignity, dominion, royalty, kingdom
Cornish (Kernewek)	myghtern, metern [mɪx’tɛrn] = king, monarch myghternes, meternes = monarch, queen myghterneth, meterneth = kingdom myghternses, meternses = kingship, monarchy
Old Breton	machtiern = king

Etymology: from Welsh mach (surety, guarantor, sponsor, hostage) and teyrn (sovereign, monarch, king) [source].

Source: Gerlyver Kernewek, Geiriadur Prifysgol Cymru

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