Who wrote "Where ignorance is bliss, it is folly to be wise"?
The quote you mention actually reads "where ignorance is bliss, 'tis folly to be wise," and it is perhaps one of the most misunderstood quotes in literature.
The quote is the last two lines of a poem written by Thomas Grey called "Ode on a Distant Prospect of Eton College." This is a nostalgic poem in which Grey longs for the days when he still lived in "ignorant bliss," not knowing about the hardships and suffering which would inevitably come.
The primary theme of the work is that suffering, unhappiness, misery, and death are inevitable. He is looking down at the boys at Eton College and wishing he could somehow spare them the knowledge of the pain which is inevitable in their adult lives. The last stanza reads this way:
To each his sufferings: all are men,
Condemn'd alike to groan—
The tender for another's pain,
Th' unfeeling for his own.
Yet, ah! why should they know their fate,
Since sorrow never comes too late,
And happiness too swiftly flies?
Thought would destroy their Paradise.
No more;—where ignorance is bliss,
'Tis folly to be wise.
Grey is not, as is commonly believed, supporting the idea that ignorance leads to happiness. Instead he wants young people to maintain their blissful innocence (ignorance) as long as possible before the difficulties of life consume them.
As the previous contributor rightly noted, the line is from "Ode on a Distant Prospect of Eton College" by Thomas Gray. "Prospect" in this context means "view," such as one might have from a tall hill or mountain top, for example. Indeed, that's one of the two meanings of the word as used by Gray in the poem. He often used to to visit his relatives in the countryside surrounding Eton College, and on his travels, Gray will undoubtedly have enjoyed the majestic view of his old school in the middle distance.
But the prospect is distant not just in space, but also in time, as Gray casts his mind back to the carefree joys of youth. At school, the boys are ignorant, not in the sense that they don't know anything, but rather in that they are wholly innocent of the ways of the world. In such a context, wisdom is not related to book smarts, it simply refers to a knowledge of the wider world outside, an often scary and confusing place for children of such tender years. That being the case, it would indeed be a "folly to be wise," to concern oneself with the harsh world outside when there's so much fun to be had playing games and reveling in the joys of youth.