I have come across such a phrase" unreligious to the point of irreligious". What's that about? Does it mean that ir is stronger than un ? I thought these suffixes meant the same :(.
Not quite. They can (in some contexts) be so close in meaning as to be interchangeable. But the difference between unreligious and irreligious is similar to the difference between amoral and immoral: 'having nothing to do with religion/morals', as against 'behaving in a way contrary to religion/morality'. (Fundamentalist believers that 'he who is not with me is against me' would say that they both amount to the same thing; most language-users would disagree.)