Düzeltmeler

If I want to say that I lived somewhere 5 years ago I would say: I lived in Ireland 5 years ago... but if I want to say that I was there for a 3 years, should I say - I have lived in Ireland for a 3 years, and also... why can't i just say I lived in Ireland for a three years if we don't know when it has started..

I am only telling you for how long.

So why present perfect??? Thanx!