As you probably know, many languages have two different second-person pronouns ("you"), one formal and one informal: German "Sie" and "du", and so on. English used to have this distinction too but has lost it. More here: http://en.wikipedia.org/wiki/T-V_distinction

What surprises me is that, while Irish doesn't have this distinction, Welsh does (formal: "chi", informal: "ti"). So what about the other Celtic languages? Do they, or do they not, distinguish between formal and informal "you"? Did Irish at some stage have this too?

Nó an bhfuair na Coimrigh ó na Sacsan é? (Bhíodh Thee agus Thou sa Bhéarla, ní húsáidtear anois ach amháin do Dhia é...)

Wikipedia says that Scottish Gaelic also has this: "thu" is the informal, "sibh" is the formal (as well as the plural).

Irish Gaelic does have informal and formal 'you'.

While now only used to address such people as the clergy etc. it is the same as Scottish Gaelic: The singular Tú/Thú as the informal and the plural 'Sibh' as the formal.

Cá háit? Níor chuala mise riamh é!

Léigh mé (sa Bhéal Beo, sílim) go raibh nós ann beannú do dhuine aonair le "Dia Dhaoibh", ach toisc go raibh tú ag beannú don duien agus dá aingeal coimhdeachta.

Cloisim Timmy ar Cois Cuain ag rá "Dia Dhaoibh" leis an tSagart, ghlac mé leis gurb sin an chúis.

Seanchleachtas a bhí ann gan aon amhras agus tá sé le fáil go fóill i measc cainteoirí áirithe i nGaeltacht Thír Chonaill cibé ar bith.

Cluintear 'Cad é mar atá sibh?' agus daoine ag caint le heaspag srl.

Suimiúil. Níor thug mé an úsáid sin de "sibh" faoi deara riamh, ach beidh mé ag faire amach dó feasta!

Tá mé réasúnta cinnte nach raibh an rogha sin idir ómósach agus neamhómósach ar fáil sa tSean-Ghaeilge maidir le tú, tussu ar láimh amháin agus sí, sib ar an láimh eile. Ná i Laidin ach oiread, más buan mo chuimhne, nuair a bhí na Rómhánaigh ar bhuaic a réime. Níl a fhios agam céard a tharla sa teanga sin sa Mheán Aois, áfach.

Both formal (vous) and informal (tu) in French.

In Breton: it depends on the dialect. Some have a "thou" (te), other have only "you" (for formal & plural) (c'hwi). Thou & you in Leon, Tregor, a part of Cornouaille and the most part of Eastern Vannetais. Only "you" in the other part of Cornouaille and in Western Vannetais and a small part of Eastern Vannetais.

In Welsh, you can mention "chdi" as well, which is a blend of "chi" (you) and "ti" (thou).

In Scottish, there's a formal pronoun (sibh) and informal (thu), unlike Irish. In Manx it's as in Scottish (formal: shiu, informal: oo).

In Cornish, according to Williams' dictionary, you can only say "ty" (=thou in Cornish) when talking to one person.

You're right Dennis, in Classical Latin, you only used "thou" (tu) when talking to one person. People said to Caesar "Ave Caesar, morituri te salutant" (Hail Caesar, those who will die salute thee).

Go raibh maith agaibh as an eolas sin. De réir mar a fheicim, bíonn an nós seo ag teacht agus ag imeacht ar feadh an ama.

Da hat Er recht!

Bhí aithne agam air leaid óg amháin, a raibh mímheas aige ar Ghaeilge na hAlban díreach mar gheall air an dífear seo sna forainmneacha idir na Gaeilgeacha (ní raibh aon locht ar a chuid Gaeilge na hÉireann an t-am sin, caithfidh mé a rá): chuir sé síos do thionchar Sasanach é gur spreagadh an fhoirm ómósach isteach sa gcaint, agus mar sin ní bheadh aon deileáil aige leithi, mar bhí an ghráin aige ar Shasana. Fear principled a bhí ansin, i ndáiríre.