I've a similar problem and come to similar conclusions - that neither Section 511 TCA nor Chapter 10 of the Tax manual doesn't shed a lot of light on the first-in-first-out rules for CGT, while the shares in APSS remain in the unavailable state.
My problem is the opposite - does any sale of any block of stock acquired after the APSS appropriation date trigger CGT based on the price of stock appropriated in the APSS, assuming no earlier acquisitions exist. I don't believe so on the basis that stock inside the APSS are not "marketable shares" until the release date. Section 580 TCA doesn't say that specifically but the Tax and Duty manual on Section 581 from Revenue does (I can't link to it). They use the term "Disposal of marketable shares".
They aren't marketable if I don't have access to the shares to sell them - under APSS rules they are held in a Trust account in the name of the Trustees. I still pay tax on the dividends but those are released to me explicitly.
As AAAContributor says, the base cost for CGT is that of the date of appropriation (the "Locked In" price) and that's definitely true given the linked article. However I think it's more reasonable to assume the release date is the date of acquisition for CGT.
Which implies all sorts of things, not least a sale of a block of shares within 4 weeks of the release date is considered to be a sale of the released shares, not that of an earlier acquisition.
I asked my HR department and they said "don't do tax advice". Thanks guys. I also asked the Trustee company and they said "CGT is calculated using the Locked Value (purchase value) and the Net Sale Value". Super. I assume that means they don't know.