Thanks Brendan. Carrying AVCs forward for future tax relief is definitely possible - Form 12 has an explicit box for this (Amount carried forward from a prior year, from which relief has not been obtained under AVCs) and I've definitely seen references to it on revenue.ie and third party sites.
As to why I'd want to do this, my pension is woefully underfunded: my fund is less than my annual salary as I deliberately prioritised other financial goals in the past. I now want to maximise the "time in the market" of my future contributions. What prompted this line of thought was that I've just made a back-dated AVC for 2017 and realized that I've missed out on all the equity growth since then by not contributing at the time, which got me thinking about future contributions.
I expect the majority of my income to be taxed at 40% for at least the next 10 years, even allowing for a drop in income should my circumstances change. My skills are very much in demand, and I have a generous income protection policy should I be permanently incapacitated.
Another way to think about this - I have a lump sum (say 5 years worth of AVCs) which I want to invest in equities. The most tax efficient way seems to be to put this into my pension scheme now, and claim relief over future years (not just 2019). I can get tax-free exposure to equity growth now (accepting the risk that markets fall) and use it to increase (via tax relief) my net income in future years. Gross roll-up and the AVC lump sum make the pension vehicle attractive for this even though it's really income tax deferral, not tax relief.
One issue: if I contribute 5 years worth of AVCs to my employer scheme now, but then change employer before the 5 years are up, the past contributions might not (legally) qualify for tax relief against income from another source (that seems silly to me but may well be the case). This might be an argument in favour of using a PRSA AVC vehicle, perhaps self-directed (fees are not really a consideration as my current employer scheme is actually very competitive, although I might prefer a wider choice of funds.).