"1. Why on earth did the entire legislation have to fall if a small part of it was found to be unconstitutional?
2. If there is a valid reason for this, why have entire Finance Acts and other laws not similarly fallen by the wayside when elements thereof have been declared unconstitutional?"
The entire legislation did not fall, and there is no such inconsistency. You can read the judgment of the Supreme court, but the relevant part of it simply states:
"I would allow the appeal and grant a declaration that s.1(1) of the Criminal Law (Amendment) Act, 1935, is inconsistent with the provisions of the Constitution."
Section 1(1) is the section which states:
"Any person who unlawfully and carnally knows any girl under the age of fifteen years shall be guilty of a felony, and shall be liable on conviction thereof to penal servitude for life or for any term not less than three years or to imprisonment for any term not exceeding two years"
I am a little intrigued by the phrase "unlawfully and carnally". It occurs to me that the state could have argued that use of the word "unlawfully" must have been intended to mean something, as opposed to being mere surplus verbiage. If you accept that it means something, what does it mean? Could it have been used to distinguish the situation of an "innocent" transgressor (i.e. one genuinely deceived about a girl's age, who it might be argued was not therefore acting "unlawfully") from someone who had a culpable\criminal knowledge? It's a big stretch, and almost certainly one which would not have passed the court's scrutiny, but I am a tad surprised the state didn't try it anyway. Truthfully, however, the state was on a hiding to nothing.
It is very easy at this remove to blame legislative lethargy, but part of the larger truth is that we get the politicans we deserve. While the legislation in question has been commented on before, it was almost always the subject of comment by lawyers in a legal\academic capacity. I don't think it was ever regarded as good ground for a political commentary. Who honestly believes that - absent the recent court case - any politican in this country would have found it a good career move to sponsor amending legislation? Who honestly believes that our gutter media would not have portrayed this as being a softening of position on child abusers? To the extent that this society of ours doesn't seem to facilitate reasoned, in depth debate on complex, nasty, awkward issues, we all share some of the blame.
Of course, to the extent that our prosecutors relied on prosecutions brought under a dubious law, they undoubtedly take some share of the blame. But- as a general principle - it is not fair to state that prosecutors or the state should have somehow known all along that the legislation was unconstitutional. You can't know such a thing until the legislation is challenged. I suppose you might say that our laws are entitled to a presumption of constitutionality, and those charged with upholding the law are entitled to proceed on that presumption.