Go online and you'll find places similar with much the same address and that should give you a market valuation, when you sell it you'll find out its true value. Or maybe give us an idea here about the apartment without telling us exactly where it is.
Can a non-estate agent legally provide a valuation of a property?
In any case Dublin city centre apartments sell all the time. There is very likely one in the same block listed for sale or on the PPR which could be used for a good comparison.