Visa on arrival

Hi there, do Irish Nationals need to apply for visa in advance or can get on arrival? We arrive morning time on 1 July and depart evening on 15 July so technically it is exactly 15 days. I note your comment below that if LESS than 15 days visa can be got on arrival by British National. Is it the same for Ireland (not part of UK)? Is 15 day duration ok or has to be less than 15 days?