There are limits to the amount of AVCs that can benefit from tax relief. If over age 60, that limit is 40% of Salary each year. You can contribute more, but it may not benefit from tax relief, which would not be a sensible strategy.
In terms of lump sum AVCs (often called “last minute AVC”) you could invest a lump sum prior to retirement and if that exceeds 40% of Salary in the last year, any excess over the 40% can be spread back into last year and a refund of tax can be applied for. But you can only backdate into the last tax year.
So if you were retiring in say Dec 2019, then a lump sum invested in say Nov 2019 could initially be set against your 2019 tax bill. Any excess could be spread back into 2018 (again subject to the 40% limit).
AVCs can be a very tax effective investment strategy particularly where the AVC fund is being established in order to maximize the tax-free retirement lump sum. So tax relief on the contributions and getting the AVC back as an additional tax-free lump sum on retirement (because the main scheme is not providing the Revenue maximum, perhaps due to short service).