I think the main issue here is not being discussed at all. The main issue is that certain parts of west Dublin are blending or fusing into each other. If you have two locations with similarly built houses, organised in similarly designed parks and estates and one of these areas doesnt appear to have too many additional advantages over the other, then clearly and over time there will be a convergence of house prices and attitude towards the overall area. Much to the annoyance and chagrin of many Castleknock residents, this is now happening in that area also and will continue to do so. It would be foolhardy to think that this has been caused by the "misrepresentation" of estate agents . This is complete nonsense. Canny buyers will inspect both sides of the "border" and unless they find something particularly special or particularly distasteful on either side of the border, then prices will converge over time. The cost of building a 140 sq metre home is the same on both sides (currently about 200,0000) If the finished house in Blanchardstown is costing 300,000 and the finished house in Castleknock is costing 500,000, then the Castleknock buyer is paying three times more for the site than his counterpart.
The mistake (or trap) that some people fall into is not realising that perceived or factual borders between desirable and less desirable areas are not guaranteed or fixed and are subject to change over time. The name Castleknock is just a label to describe an area of certain coordinates on a map which can expand or contract at any time at the whim of the local council . When you decide to pay an extra 200,000 for the name Castleknock, this is an intangible that takes place very often in one's head and has no basis in law. Right to a location name is certainly not protected by the law, even if you have paid 200,000 extra for it. I think the question that the OP should be asking himself is whether it was wise to pay Castleknock prices in the 1st place.
Dont forget that markets are sophisticated and prices are set by the market and by the interaction of supply and demand in a particular area.An estate agent doesnt set the price. At best he might be able to influence the perception towards a particular area but he is perfectly entitled to do that.Unfortunately, You have no exclusive right to the name "Castleknock" Its not yours, you didnt pay for it. Its just a label for the place where you live. Your thought process is not uncommon but it is flawed.