For the money that 156's cost in Ireland, I'd look at importing one from the UK, and paying the VRT - you'd save a packet.
I've exported 4 cars from here now for family & friends, and every time it has saved them about €3000 all in - even with the Euro/Sterling exchange rate being the way it is.
There are a number of dealers in the UK off-loading 156 selespeed 2.0 ts models for £10K - £K12. "Brand new" 2008 plated, fully loaded, with 3 year warranty, if you plan to keep it for a while could be worth a look.
Nice idea, but the Irish VRT would screw the price way up - so £10k for the car, another £5k in import duty (VRT)... not worth it.
You pay less duty for an older car / more mileage... so for a 2004 156, it wouldn't be much - you can find the car on Auto Trader UK - Buy & Sell New & Used Cars, Car Loans, Car Insurance
, then get a VRT quote on www.ros.ie
Road Tax is a rip-off in Ireland too, as is insurance, hence the probable need to stick to a 1.6. According to the VRT website, the car in your link above is only worth about €9500.
Just Sold: 2001 156 2.4 JTD (140) Sportwagon Veloce
Replaced by: 2004 (54) Saab 9-5 2.2TiD Linear Sport
Also own: 2007 (07) Vauxhall Astra Twintop 1.6