Assuming it's not a PRSA, your employer contribution isn't counted.
You've already paid 6% of your base salary (74k) = 4,440
You can get relief on 30% of total pay (81.4k) = 24,420
So you should be able to contribute up to a further 19,980 for last year and get tax relief on it.
You should have contact details for your pension provider. If not, your HR should be able to help you.
For me, I contact the provider and tell them I want to make a back dated AVC to maximise tax relief. They ask for a copy of my P60 (or whatever it's called now), and they calculate the maximum contribution I can make. They then send me bank account details and a reference number that's unique to me. I transfer the gross amount of money, and they send me a certificate to claim tax.
Then, I log into Revenue, and add the details in my tax details for the previous year. They recalculate a balancing statement, and transfer the tax relief back to me within a few days.
Your HR department should be able to help you with the full process.