The spellchecker on this site (US spelling mind you) can probably be downloaded - did you mean a spellchecker generally or for this site? Either way you can probably use it.
I'm pretty sure the spell checker is in your browser, not on the site itself. So IE Spell is within your Internet Explorer browser, and will work for all sites you visit. There is a similar spell check facility in Firefox.
OP - What browser and what version of that browser are you using?
Do you mean when using eircom webmail? There's a spell check button on the toolbar when you are creating a new message (just above where you enter the sender's address).