API NG Sample Code Help - marketFilter.MarketCountries

Hi, I'm using the sample code (VB.NET) to try and load markets from GB and Ireland, but I can only do one at a time, I'm sure I'm making a basic error but not sure what it is:

This works:
      marketFilter.MarketCountries = New HashSet(Of String)() From {"GB"}

But if try and and Irish races it still only loads the GB races:
      marketFilter.MarketCountries = New HashSet(Of String)() From {"GB", "IRE"}

Any help appreciated.