Lenition, known as séimhiú in Irish, is a particular feature of the language. It generally makes speaking easier and more natural (which in itself is an indication of when to apply it), but it comes at the cost of grammatical complexity.
Séimhiú is indicated with a letter h after the first consonant of certain words, usually nouns and adjectives. In the default state of the word the h is not present (this is the version you will find in a dictionary); rather it can be inserted as a result of what comes before it in a sentence. In most cases, one does know when to place this h. However it does happen that even experts have doubts in some cases.
The following document (in Irish) is available on-line, and it gives a brief outline of when and when not séimhiú should take place. If you need to read this document, your Irish is probably good enough to understand it.