According to Tourism Ireland 40% of Irelands inward tourists are from the UK. The US accounts for something like 15% or so. However, the US account for approximately 25% of the total spend in country.
Putting aside the worldwide recession, the Euro has been strong against both the greenback and Sterling over the past 2-3 years. Its only in the past 6 months or so where it has lost ground. The poms are a fickle bunch as well. They are happy to travel when they are getting 2:1 and the like for their money but don't like it much when sterling is weak.
This isn't about a 10 quid tourist charge. Its about currency issues, rip off Ireland, a worldwide recession and the fact that there are better tourist options out there that Ireland has to compete with.