in alping - oldest parliament - who sat left, who right of speaker?

In the modern Althingi, seats are assigned by lot, if that is any help.

http://www.althingi.is/pdf/enska.pdf

There doesn't appear to have been a division of the assembly originally.

hlabadie-ga

You might want to read the parts of Njal's Saga that concern the
attendance at the Althing, where law suits were prosecuted:

http://www.northvegr.org/lore/njal/059.php

The litigants seemed to arrange themselves north and south of the
Law-Speaker, but primarily because that was where their allies were
posted in anticipation of combat.

hlabadie-ga