ITV and C4 have commercial arrangements whereby they're paid by Irish cable operators who wish to include those stations in their line-up. However that means that, when purchasing programmes from production companies, they have to pay for the Irish broadcasting rights, as well as the UK ones.
Channel 5 doesn't have such a commercial arrangement, so it doesn't purchase the Irish broadcasting rights from production companies.
Further, there are probably historic reasons why a Sky box might have problems with receiving Channel 5. Prior to 5th November 2008 the station was unusual in that it was 'free to view', rather than 'free to air'. While 'free to air' channels can be received through any suitable satellite box (because they're unencrypted), 'free to view' channels require a box with a valid card inserted (to decode the encrypted signals). So the way that a Sky box handled Channel 5 signals was always different to the way in which it handled most other channels. (I suspect that the reason why the signals were originally encrypted was precisely to ensure that only UK Sky cards would work, rather than Irish ones, because of the broadcasting rights issue I've referred to earlier).
Now that Channel 5 is 'free to air', it should (despite the issues over broadcasting rights) be available anywhere within the footprint of the Astra 2D satellite by using a 'basic' digi-box (which is possibly why you've been advised to take the card out, as its presence might result in the box blocking a channel which used to be encrypted). The information you require for manual tuning is in the right-hand column here:
http://en.wikipedia.o...channels_at_28%C2%B0E
Chris