I'm interested in joining the British military?

Hello, I'm 15 years of age, I'm originally from Latvia but I'm living in Ireland. I was reading trough the UK army website but I need some points to be made clear. First of all, I require to have lived in the UK for at least the duration of 5 years? Because I don't seem to get the point of that. I have an EU citizenship (Since Latvia is in the EU...), do I need to get a British one? Can anyone point out the details in full, like what I require etc. Thanks.