[Nuranar]

I don't know, Willie. For your first, it may come down to what Gollum's true intentions are - and that in itself may be difficult to determine (i.e. Why did he follow Frodo and Sam? Was he planning to get the Ring back, or was he just watching to see what Frodo did with it?) But I do think that when he attacked Frodo after Frodo claimed it, Gollum was trying to take the Ring for himself. At that point, since Frodo had claimed it, the Ring was in no danger of being destroyed.

However, I think a case may yet be made for irony: Whatever Gollum's intentions were, his actions were calculated to cause anything but the destruction of the Ring. That in the end, he was the instrument of its destruction, is probably irony - "incongruity between the actual result of a sequence of events and the normal or expected [or intended] result."

As for Merry's sword, though...that is not irony. It is a wonderful plot development, more of a grand poetic justice: A blade wrought to wield against Angmar would, a thousand years later, be the downfall of its fell king. It also may illustrate the power of those who wrought it and how, although they were gone, their legacy was not ended. And under the One, there could be no coincidence.