Can't advise on AVCs vs purchasing notional years of service etc etc but compared to nothing, yes AVCs are a great idea for you. You'll be able to contribute 40% of your salary before tax
AFAIK you can use a portion of your AVC fund to top up your lump sum to the maximum allowed (1.5 times salary). It could also be possible to utilise some of the AVC to buy back years just prior to retirement but if you are only starting your AVC now it is unlikely to become a very considerable fund in just a year and a half. Alternatively you could look at buying back years but it likely to be very expensive attempting to buy back 4 years with only 1.5 years of service remaining. Of course you can also do the math on the relative cost/benefit analysis between a full 40 year pension entitlement v's a 36/40 entitlement (80%). No harm in chatting to a financial advisor to explore your options.
UPDATE: Just came across post from sidzer in this thread which is not dissimilar to your own situation so AVC's are probably your best option
If someone earns €100k a year, they’re 42, they’re a pre-2004/post-1995 entrant with an NRA of 60, and they’ll have 39 years’ service at age 60, approximately how much would it cost to purchase the missing piece via a single lump-sum? By my reckoning it would cost €30,000. (Appendix 2, Table 6...