> "Undir því væri þá," segir Gunnar, "að eg hefði málaefni góð."

I think Grace has the right idea here. If I've understood correctly:
'væri' "it would be", 'undir því' "depending/dependent on that", 'þá'
"then, in that case", and 'því' = 'at ek hefða málefni góð' "that I
have (subjunctive) a good legal case". "it" = "how things fare"? --
hence MM & MP's "then it will be important that..." See Zoega's 3rd
definition for 'undir' "depending on". Another one of those sentences
where a subordinate clause is prefigured by 'þat', here dative [
http://tech.groups.yahoo.com/group/norse_course/message/7496 ].

LN