3 Deirdre

Deirdre is thus far the only unit with spd ploy, so I'd only merge 1.

+atk -def should be optimal for her. I'd keep that and +spd - hp, but at a glance I'd say the speed won't help nearly as much as the attack and her base res with DNaga is high enough to already tank a +atk 40+10 Rein w/hone cav, and death blow. They don't get much tougher than that

I'd go with +spd/-hp, I think that the +spd helps her incredibly much in combination with spd ploy, and 35 res is already high enough. Alternatively, I'd pick the +atk one, +res/-hp feels like an overkill for res.

Without a doubt +atk/-def should be the one you use 31 speed doesn't save you from many doubles at all nor should you be using her to tank anyway so an offensive nuke with +atk and using something like iceberg or glacies would be optimal. Also I'd hold off on merging the other 2 as spd ploy could be a very useful skill to over units with good res who can utilize it.
With her canceling any buff during combat she shouldn't really need the speed for most matchups either so yea hope that helps and gratz on your deirdres

They are all good options, but I'd like to add the counter argument for +res/-hp. While yes, her already high base res makes it seem moot to go with +res, I wouldn't say Deirdre's main role in a team is as a primary attacker. She can easily bait more non-red magic threats with +res, and it adds to the damage output of iceberg if you choose that special.

+atk/-def makes her closer to a neutral atk Julia, but most green mages don't need that high of an attack to deal with meta blue units. You're better off baiting those magic threats out with Deirdre's high res and QR.

+atk/-def does have the benefit of throwing away her most useless stat though, whereas losing more of her fairly low hp in both +spd and +res boons is a little rough.

How you got 3 I will never understand (75 orbs, no 5 stars for me) but It really depends on what you need I think. If you want a raven build don't touch her def. It's already pretty low. If you are going TA no raven, (My plan) then +atk maybe? She will never double blue mages (At least I don't think) and atk is necessary. Ploy running would be good if you want the speed ploy to get almost everyone with the +res. It's a tough choice but good luck.

Go with +res and please don't raven her. Do that with+attack julia. Unless you want to make your +attack deride raven.