Thank you, Sinéad. I agree with what you said too. I have seen no Galvins mentioned in any Norman documents.
As a followup, after studying Irish for some months, slowly blundering my way through the pronunciation, my teacher gave us what he calls a "decoder." This could help regarding the argument about whether the name ends with IN or AN.
The name in Irish is spelled Geal?áin or, for keyboards without the dot over the B, Gealbháin.
My decoder says the vowel combination ái is pronounced AW-IH (I as in "it"). My other Irish teacher says the combined vowels are said quickly, barely opening the mouth.
So I assume the VAN pronunciation, with the A like "cat," is not authentically Irish. It's VAW-IN, said quickly.
Nobody has yet burdened the speakers at Forvo with this name, but we should! Then we'll all be happy. You'd be astonished at the differences between what we think we know as our name and what the Irish is. Murphy, for example, is O Mur-a-hu, or, if you're female, Ni Wur-a-hu.