The conjugation of irregular verbs — essential for learning English.
Literal
English [object-を] study [in the course of-上で] indispensable, irregular-verb [possessive-の] conjugation.
~する上で 'in the course of doing, when doing' is a formal pattern. 不可欠 'indispensable' and 不規則動詞の活用 'conjugation of irregular verbs' are standard educational terms. The sentence is a fragment — a heading or topic introduction with no main predicate.