In my experience, the plural form "agonies" is typically seen only in literature.  It feels more expressive because we can imagine that the period of agony involved varying types of agony. In general, we use "agony" as an uncountable noun in everyday speech and writing.

Regarding Mr Lance's comments, I agree with his main point at the beginning.

"Coffee" as a countable (cup of coffee) has a different meaning to "coffee" as an uncountable.

"The staff IS" is always wrong. "Staff" is a unusual noun.  It always refers to the group (as opposed to "a member of staff").  "Staff" has a singular form but takes plural verbs. So only "the staff ARE" is right. Another word like this is "police". 

Hi Jude, 

This is a noun that can be countable or uncountable, depending on the context.

If we are referring to the state of physical and mental discomfort, it is uncountable.  (Example: He screamed in agony at the broken body of his child.) If we are referring to various types or episodes of agony, it is countable. (Example: The agonies of war.)

Just as a side discussion, some nouns can be countable or uncountable, depending on their usage.

Example:

Coffee (as a beverage is uncountable): Coffee contains caffeine. 

Coffees (considering different types of coffees): May I have three coffees, please?

Some collective nouns do not have a plural form, but can be used to describe the collective whole or each element in a group.

Example: The staff is working overtime tonight. (all the employees are considered together as a group) / The staff are shocked to hear about the layoff. (each individual employee's response)

Reference: http://ontariotraining.net/grammar-question-all-staff-is-or-are/

Hope this helps.