What is the terminology for a phrase whose common usage is diametrically opposite to its literal meaning?

For example, consider the following simple phrases one typically says upon entering a popular restaurant:

• I love chicken.


• I love fish.

If the person literally loves fish, he/she wouldn't really want to take its life and eat it. What they really mean to say is I love cutting and eating a fish.

Does this come under Irony? But Irony is generally a deliberate attempt to reverse a meaning for creating humor.

Answer

If one word means its opposite, it's called an autantonym or contronym.

What you are referring to is called polysemy:

(from Greek: πολυ-, poly-, "many" and σῆμα, sêma, "sign") is the capacity for a sign (e.g., a word, phrase, etc.) or signs to have multiple related meanings (sememes), i.e., a large semantic field. It is usually regarded as distinct from homonymy, in which the multiple meanings of a word may be unconnected or unrelated.

Charles Fillmore and Beryl Atkins’ definition stipulates three elements: (i) the various senses of a polysemous word have a central origin, (ii) the links between these senses form a network, and (iii) understanding the ‘inner’ one contributes to understanding of the ‘outer’ one.

Love can be used reasonably and accurately with both versions you give, because language is more than the sum of its parts. Love is much more than one thing, though one word can be used.

• I love fish. I have raised tropical saltwater fish for 12 years now.


• I love fish. The light, flaky texture and delicate flavor of (favorite fish here) beats steak any day!

Verbal irony is something different. After the Charlie Hebdo attack, verbal irony was everywhere, as in this cartoon.

Edited to add: OP's comment is an example of contronym:

Contronyms ...do not tend to cause confusion, as it is generally clear from their grammatical or semantic context which of their two meanings is intended. In the sentence “He buckled,” for example, we know that the subject collapsed, since in its sense of “to fasten,” the verb is transitive and must be followed by a direct object; similarly, in the two grammatically identical sentences, “The house weathered the storms” and “The storms weathered the house,” simple logic tells us what “weathered” means in each case. Yet there can certainly be instances — e.g., “The horse had a handicap” — in which a contronymic sentence is interpretable in opposite ways.

_{Till a' the seas gang dry, my dear,/And the rocks melt wi' the sun;/And I will luve thee still, my dear,/While the sands o' life shall run.}