How to avoid perma-war?

[athanaric]

My advice would be to conquer all of Iberia, so that you get peace on one front at least.
If you don't want to eliminate the Lusotannan faction through this course of action, you can force-migrate them to Ireland.
This is possible without Force Diplomacy, and I removed Baktria to Krete this way in a campaign (Saka) where I needed their land, but not their lives, for Victory Conditions.

If you want to do this, land a diplomat on Ireland first. Bribe a town there (preferably Ivernis), and teleport most, if not all, Lusotannan stacks there (especially those with generals). Now if these don't attack and take your Irish town right away (because they hate you), "gift" it to them. It might be necessary to give them an additional 1 million mnai or more to have them accept.
After they are settled in that town, immediately take their last Iberian holdings.

And don't worry about their not getting factional units on Ireland - the Lusotannan get excellent units there (even without the two campaign-disabled Wunderwaffe units).