What would you like to do?
Since there are six distinct Celtic languages there would be six different words.
The Celtic language with the most speakers as their mother tongue is Welsh so I will assume you are asking to say it in Welsh - "Nos da". In the other Celtic languages it is "…Noz vat" (Breton), "Oíche mhaith" (Irish Gaelic), "Oidhche mhath" (Scottish Gaelic), "Nos dha" (Cornish) and "Oie vie" (Manx).
In Irish it's "dragún / dragan" In Welsh it's "draig" In Scots Gaelic it's "nathair sgiathach / dràgon"
Perhaps this information on "Answers.com" may assist you in learning and forming your own deduction: http://www.answers.com/topic/pronunciation-of-celtic
Irish: abhainn Scottish: abhainn Welsh: afon Breton:
In Irish it's: madra / gadhar In Welsh it's "ci" In Scots Gaelic it's "cù"
In Irish it's "aonbheannach" In Welsh it's "uncorn" In Scots Gaelic it's "aon-adharcach"
Irish: draíocht Scots Gaelic: draoidheachd Welsh: hud; dewiniaeth; swyngyfaredd
In Irish it's "sliabh" In Scots Gaelic it's "beinn" In Manx: In Welsh it's "mynydd" In Breton: In Cornish:
In Irish it's "síocháin". In Scottish Gaelic it is sìth In Welsh it is heddwch (also tangnefedd)
cairde (Irish) càirdean (Scottish Gaelic) cyfeillion, ffrindiau (Welsh) karidi, keveiled (Breton)
Irish: ansiúd/thall (adverb); thar; lastall de; taobh thall de (preposition) Scots Gaelic: thall; thairis; seach; thar Welsh: tu hwnt
There's actually no such language as "Celtic". Celtic refers to a group of dozens of languages, six of which are spoken today: BretonCornishIrish GaelicManxScottish GaelicWel…sh
Celtic is a language family of six distinct languages: Irish = fánaí (Scottish) Gaelic = iniltear Manx Gaelic ... Welsh = crwydrwr Cornish ... Breton ...
Irish: ceannairc; éirí amach; reibiliún Scots Gaelic: ceannairc; aramach; reubalachd Welsh: gwrthryfel