PTSB & ESRI produce a quarterly house price index going back to beyond 2002:
As this is my first post on here it won't allow me to post the URL (Grrrr), but if you google "PTSB ESRI survey" it should be the top hit (to the ESRI.ie website). Scroll down a bit and there's an excel sheet you can download with the index going back to 1997.
If you know the current value (or if the property was subsequently sold, the value it was sold for) you can work back to a reasonable approximation of the value in 2002...
For example:
Assuming property outside Dublin, valuation date Q3 of 2002:
Index Q3 2002: 90.1
(Say) Sold in Q3 2007 for €400,000. Index Q3 2007: 135.5
Therefore a reasonable estimate of the value in 2002 would be €400k x 90.1/135.5 = €266k
Or if its still on hand at a current value of say €330k, the latest index is at 96.1, so the 2002 value is €330k x 90.1/96.1 = €309k.
Thats a fairly transparent and reasonable calculatio; I know I wouldn't dispute it if it came across my desk (not least because using the index is likely to be less subjective than a retrospective valuation by an estate agent, who you can be fairly sure isn't likely to bust his/her hump to trawl through sales in your area from 2002 of comparable properties to find a realistic value - if they're smart they'd do a generalised calculation like mine above, and if they're lazy they'll pull a number out of their head and charge you for the privilege...!).