Ceart go leor. Fada ó shin - I was myself governed by the Brothers of the Christian Schools - not quite the same order, but definitely franchacha .

First of all I find the way this is expressed in English rather confusing, so trying to put it into Irish is even more so. The phrase is - "that is true for you' - here's the context:

" 'T is a fine wholesome appearance the poor children carry," remarked Robin, "although they have been so long shut out from the fresh air and the blessed light of heaven. 'T is tenderly your honour must have reared them !"
"Ay," said the giant, "that is true for you; so give me your hand; for you are, I believe, a very honest fellow for a blacksmith."

Does it mean 'coming from you' ?

My understanding is that it means: That applies to you.

So would - seo dhuit an fhírinne - be the way of saying it?

"That is true for you" sounds like a literal translation of "Is fíor duit" - "You're right/What you say is true"

Go raibh maith agat

Here's another phrase that I'm not sure how it is expressed in Irish -

"Let me live or die for it, ........(but this is young Philip Ronayne.") - and in the next sentence, how do you put in the emphasis ? - "it IS Philip Ronayne