It depends :-). Google uses a number of factors for geotargeting ( http://googlewebmastercentral.blogspot.com/2010/03/working-with-multi-regional-websites.html ), they include the TLD (are you using a .co.uk or a gTLD like .com?), the Webmaster Tools setting (for gTLDs), the hosting location (that would be what you're asking about) and a bunch of other factors.
If you're using a ccTLD like .co.uk, then you've pretty much covered it and the location of your hosting doesn't really matter (this is, I believe, valid for the other large search engines as well). If you're using a gTLD like .com, then I'd recommend using Google's Webmaster Tools to set geotargeting (I don't know what other search engines supply). Either way would allow you to host your site anywhere that is convenient to you.
Another factor to keep in mind (and one of the reasons that hosting location is used) is that the hosting location can sometimes influence the speed at which users are able to access your site. If, for instance, most of your users were in China and your site were hosted in Ireland, it might be that these users would experience your site as being slower than a site hosted in their country. I imagine this is not an issue in your case though.