"You are welcome, professor."
Translation:'S e ur beatha ollaimh.
a sheanair, not a seanair
It is just ollaimh because the vocative particle a is always unstressed /ə/ and it disappears before vowels, and ollaimh starts with a vowel. It used to be written (so you can find a ollaimh written in Gaelic) but it typically is not in modern writing.
They are, they both come from Old Irish for, far, bur, bar. It evolved into bhur, bhar in Early Modern Irish / Classical Gaelic, and then to Scottish ur, Munster úr, and Ulster mur (I guess by wrong delenition of bhur as if it were *mhur), and in Connacht it got replaced by a – although even in dialectal texts in Irish it is almost always written as bhur.
But notice that even in Irish it is not pronounced bhur today (but rather úr, mur, or a) and Munster úr is pretty close to the Scottish form. ;-)
Yes, when you speak to somebody with whom you are not on first-name terms (somebody older, or in some formal setting when you need to be polite), you use plural ur instead of singular do, the same way you use sibh instead of thu, agaibh instead of agad, etc.
(But as noted in OP’s comment – not in Irish, there you always use the singular tú, do, agat, etc. when speaking to one person, and plural sibh, bhur, agaibh, etc. only when speaking to multiple people.)
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