IMO, the main advantage of a .COM address is that people tend to try it first. If the .COM equivalent of your (proposed) .IE site operates the same type of business, then potential customers may end up there by mistake.
Of course this also depends on how you will be getting people to your site - if they're clicking on a link, the name isn't too important, if you're relying on Google, then it may be. For a worldwide audience, I think a .COM is better.
Where possible, I think it's better to own both the .COM and the .IE version of your chosen domain name. Then you can make a decision as to which one will be your primary site. You can set up a dummy site, or a permanent redirection from the other one.