Agree it's subjective. But if it's by an Irish artist, they guys in De Vere's and Whyte's have been in the business for years. They would have all the price information.
For example, "A similar work of the same size and quality by the same artist sold in these rooms for £10,000 in 1995"
Revenue could/would hardly argue with that.
It would depend on the sale price today though. If it's a Warhol selling for €50m, they might allocate resources to it.
But most Irish artists and works owned by Irish collectors, sell at auction for €1,000 to €10,000 today and probably not a lot more than they were worth in the mid 90s.
Brendan