Most of the Irish search engines are hybrid directories. The main player in Ireland as regards search engines is Google. The other main one is MSN and then Yahoo.
The Nielsen figures are useless drivel when it comes down to real traffic because they are based on a set of monitored users rather than on genuine traffic. Ireland is a very small market from the search engine point of view.
And just because Google has a Dublin office, it does not make them an Irish search engine. :)
Regards...jmcc