Let me set one thing very straight.... marriage was NOT 'defined' by anyone in years past. It was merely a commonly accepted 'norm'. Various institutions based on religion assumed that their moralistic leanings should be applied to everyone.... thus, in the days when almost everyone was a member of, or controlled by, some aspect of an established religion, no one even bothered to try to get married except M-F couples. It really didn't NEED to be 'defined', because sexuality itself was so proscribed that marriage was just assumed, even by homosexuals, to be limited to M-F.
   It was a tradition, not a law or definition, though some laws no doubt implied that M-M or F-F could not marry. Slavery was an accepted norm at one time also, as were other behaviors that we now find abhorrent. Slavery was a lot easier to see an abhorrent, as the evil effects were obvious and affected the subservient group(s) and society as a whole in bad ways...... marriage of two people of the same sex does not. Remember... there are some in society who could easily accept the return of slavery as a norm.

Imagine meeting someone you know is gay, but with whom you have a cordial relationship...perhaps at work. He is a nice guy, and you chat about this & that. Now imagine he mentions that he and his partner just got 'married'. What changed? His new status does not affect you in the least. His being married does not affect YOUR marriage... and the only real effect on society is that there is less bureaucratic nonsense about property, inheritance, power of attorney, contracts....etc.

Once more.... the right to marry is now...in most countries... a civil contract regulated by law in order to keep certain civil affairs orderly. Some people... even many people... also **LIKE** to attach a religious element to it and 'marry' in a religious ceremony, but the civil license is the binding one. People can, and often do, get married in a purely civil ceremony. Some churches find this to be 'disturbing', because it limits the authority that churches once held over almost every aspect of life.... but the carryover 'tradition' of limiting marriage to M-F has still clouded the civil ceremony...until recently.
   People are naturally startled to see such cracks in a 'tradition', but once they see that the only problem with the new order is within their own heads, it will soon just be the norm that 'marriage' just means 'joining of people who love each other'. Those who wish to believe that a God is involved in their own case may keep believing that- it doesn't affect ME!

The whole objection to gay marriage is a psychological leaning defended by straight people using convoluted linguistic jargon designed to make what THEY do seem the norm. In a sense, it will remain the norm, as M-F will no doubt always be the majority- genetics just works like that.

It will obviously take awhile for some cultures to follow Ireland's lead, but Ireland is a breath of fresh air in a world with so much confusion and hate.