> "Veita vil eg þér mágur því að þú munt þess eins biðja að mín

sæmd sé þá meiri en áður."

"I will grant you [your request], inlaw, because [whatever you ask, I
know that] you will only make such a request as/that my honour will be
the greater for it."

"[...] only such a request as will be to my honour to grant."

"[...] only such a request as my honour will be greater then [after
granting it] than it was before [I granted it])."

'þess' "that (thing)" is the genitive neuter singular of the
demonstrative pronoun 'það' ('þat'). It, and 'eins' "only", are
genitive because the verb 'biðja' requires a genitive object for the
thing requested. Here the demonstrative pronoun is being used in an
indefinite sense: 'þess eins...að' "only for such a thing that
[whatever it may be...]"

'sé' is the subjunctive 3rd person singular of 'vera' "to be".