given the vocative case that changes how some names are pronounced ie

Padraig ----- A Phadraig

and given that not everyone in the world has a name that is instantly recognisable , nor whose spelling could be automatically guessed at , lets say you are introduced to someone who has a name you have never heard before what is the protocol?
Do you guess? Do you ask them how to spell it?

"John, this is Mary. Mary, this is John"

"a Sheáin, Seo Máire. a Mháire, Seo Seán"

Everything's fine and dandy there.

I presume you're on about over-hearing the likes of:

"a Sheáin, tar anseo."
"Fiafraigh de Sheán cá bhfuil sé ag dul."

The ONLY thing you can do is consider the grammatical rules of Gaeilge and think what the name could be. For example, when you hear:

"a Sheáin"

You could narrow it down to:

Seán
Teán
Heán

Well, I've never been to a Gaeltacht area so I couldn't tell you! But in situations like this, the first thing I do is try to find a similar situation in my own language, English, and see how we deal with it in that language. For example, you have:

Brian
Bryan

When you hear some-one referred to as "Jay", you don't no whether their name is:

James
Jason


Anyway, as regards protocol, we're talking about a language here, so there's no such thing as protocol. You may consider that a certain protocol in English is to say "Pardon", or "Excuse me", but people are still going to say "What" and "Come again". Depends on the people.

no I dont mean the Sean Shean thing , I mean although there are common Irish names , not everyone who speaks or wants to speak Irish neccesarly has an Irish name , if I have an Indian friend who is learning Irish with me and we enter a pub and he introduces himself to you in gaelic

to your ears you hear Javinder , when in fact it is Xavinder , what do you do? Lenite it the way it sounds to you and expect them to correct it if you get it wrong , or do you ask him how it is spelled?. This is of no great importance , I was only curious as a name is something as personal as it gets and you wouldnt want someone saying it anyway the felt. If I was called gerardo by spanish freinds I would soon point out it isnt my name . I said protocol not for the language but for social etiquette reasons .

In greeting your Indian friend and responding to the SOUND of his name, I would naturally lenite his name based on the way it sounds. That's not the same thing as changing his name, simply a convention of the language in addressing anyone whose name begins with a consonant sound that is subject to aspiration. So, based on Irish orthography, I would've heard what I thought was 'Deaibhinder'. I would have greeted him as, 'a Dheaibhinder."

An example of really changing a name in Irish would be if his name were Denis and I insisted on addressing and referring to him (despite his protestation) as 'Donncha' because that would be the normal Irish translation of his name. If your friend's name were Denis, I would address him as 'a Dhenis' and refer to him as Denis using the English pronunciation of his name.

Tomás

"Me again", the written language doesn't mean shit, for want of a better word.

All forms of alteration are sound alteration. Consider "sound alteration" to be a synonym for "alteration".

For example, the Gaeilge for job is "jab". I will pronounce:

mo jab

as "mo yab". If I spelled it "mo jhab", then I would be just asking for some-one to get pendatic on me.

mo jab ( mo dheab )
ár jab ( ár ndeab )

So, in summation:

Jason -> a Jason (But I'm still going to pronounce "a Yason")

Denis -> a Dhenis