Some of these solutions would even work, for a while. So I set about finding a real solution. The de facto way to change your IP address would appear to be using some software called OpenVPN, and an account with a VPN provider. It seems there's only a very limited number of providers who have Irish IP addresses available, and owing to a speedtest available on their site, I decided to go with overplay.net. Signup was fairly simple, just the usual form, followed by entering PayPal details (it's $9.95 per month to access their UK, US, and Irish servers).
Once that was completed I downloaded some VPN software. This was just the usual process, run the installer .exe, and an icon will appear down by the clock in Windows Explorer. I right clicked this and found a list of their VPN servers ready to be connected to... next it was simply a process of entering the username/password I'd signed up with.
Finally, i visited RTEplayer, and was delighted to note that a process which took only five minutes finally allowed me to watch Ireland's world cup progress!