Words for estuary / mouth (of a river) in Celtic languages.

Proto-Celtic	*adberos = mouth, confluence (of a river)
Pictish	*ᚐᚁᚓᚏ (aber) = mouth, confluence (of a river)
Old Irish (Goídelc)	abar [ˈfʲilʲi] = confluence, river mouth
Irish (Gaeilge)	abar [ˈabˠəɾˠ] = boggy ground, morass abarach = boggy, muddy, sloughy abracht = boggy place
Scottish Gaelic (Gàidhlig)	abar / obar = confluence, place where two or more streams meet; marsh, bog, fen
Manx (Gaelg)	aber = range, pasturage, river mouth, bunker, run
Proto-Brythonic	*aber [aˈbɛːr] = mouth, confluence (of a river)
Old Welsh)	oper, aper = estuary, mouth of a river
Middle Welsh (Kymraec)	aber = estuary, mouth of a river
Welsh (Cymraeg)	aber [ˈabɛr/ˈaːbɛr] = river mouth, estuary, confluence, bay, harbour, port, anchorage
Old Cornish	aber = estuary, mouth of a river
Cornish (Kernewek)	aber = river mouth
Old Breton	aperou = estuary, mouth of a river
Breton (Brezhoneg)	aber = estuary, mouth of a river, confluence, creek

Etymology: from the Proto-Celtic *ad- (to(wards), at) and *beros (bearer, flow) [source].

Old Irish (Goídelc)	*in(d)ber = river mouth
Irish (Gaeilge)	inbhear / inbhir [ˈabˠəɾˠ] = river mouth, estuary, firth inbhearach = having many estuaries
Scottish Gaelic (Gàidhlig)	inbhir [in̪ʲɪrʲ] = estuary, river mouth, (internet) feed
Manx (Gaelg)	inver = estuary, river mouth

Etymology: from the Proto-Celtic *endo-ber-o (carrying in) [source].

Notes
These words appear mainly in placenames such as Aberaeron, Abererch, Aberffraw, Abergavenny (Y Fenni), Abergynolwyn, Aberystwyth, Abergwaun (Fishguard), Aberhonddu (Brecon), Aberteifi (Cardigan), Aberdaugleddau (Milford Haven), Aberpennar (Mountain Ash) and Abertawe (Swansea) in Wales.

Aberdeen (Obar Dheathain) , Aberfeldy (Obar Pheallaidh), Aberfoyle (Obar Phuill), Abernethy (Obar Neithich), Arbroath (Aber Brothaig), Invergowrie (Inbhir Ghobharaidh), Inverkeithing (Inbhir Chèitinn), Inverness (Inbhir Nis), Inbhir Nàrann (Nairn), Inbhir Pheofharain (Dingwall), Inbhir Ùige (Wick) in Scotland.

Aber/Obar appear used in Scottish placenames of Pictish origin, and may have been borrowed from Pictish. Inbhir/Inver appear in other placenames [source].

More details of placenames featuring Aber and Inver.

Old Irish (Goídelc)	gáeth = the sea, a stream, an estuary
Irish (Gaeilge)	gaoth [ɡeːh / ɡiːh] = inlet of sea, estuary Gaoth Dobhair [ɡiːˈd̪ˠoːɾʲ] = Gweedore (a parish in County Donegal)
Scottish Gaelic (Gàidhlig)	geodha [gʲɔ.ə] = inlet, cove, gully (by the sea), indent(ation)
Manx (Gaelg)	giau = creek, inlet, cove Giau Vooar ny Pershey = Persian Gulf

Etymology: from the Old Norse gjá (chasm, rift, crack) [source].

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