As I understand it, lenition is something which has been “evolving” to smooth the letters of the language for many centuries. What we know of lenition today is perhaps the remaining part. Simply the insertion of the letter H into the word after the first letter. Much older forms of lenition have been absorbed into vocabulary and you can see evidence of these in words like sèimh or fliuch which looked very different in the past and whose endings were lenited.
Remember the list of consonants which can be lenited from last time, not all consonants are lenited. C, B, M, D, G, S, T, P, F The letters L, N and R also lenite but are not written, you need to keep this in mind.
Below are the conditions of lenition
Choisich mi past tense *we will discuss that later, along with other tenses which can be lenited
oidhche mhath adjectives with feminine noun
mo bhràthair possessives mo, do and a, *only these possessives, always lenite. Possessives also cause the word to insert a vowel near the end of the word, Its called slenderisation. We will talk about that later.
bho, de, do, fo, gun, mar, mu, ro, tro prepositions which cause lenition.
corra deagh, droch, priomh, sàr, seann adjectives which lenite
glè mhath modifiers glè, fior, ro
aon, dà, a’chiad these numbers lenite
bu this verb lenites
cha negative particle lenites.
Kilbirnie & Beyond 