This message covers lenition and eclipsis. The purpose of both lenition and
eclipsis is to indicate a relationship between the lenited or eclipsed word
and the one(s) that came before it. Please recognize that people come to
understand these usages over years of time, and that you are expected to
have some familiarity with lenition and eclipsis, in general.

1. Lenition. When a word is lenited, the initial consonant is usually
followed by an h. In the case of vowels, the h comes before the vowel. L,
N, and R (along with sc, sf, sm, sp, and st) are never lenited. Here are
the cases in which lenition occurs:
* for words following possessive pronouns (1st, 2nd, and 3rd person
masculine), such as the example of mo chara (my friend), do chara (your
friend), and a chara (his friend). 3rd person feminine does NOT lenite, so
it would be a cara (her friend). Note that when the noun starts with a
vowel, as in "athair" (father), it's m'athair (my father), d'athair (your
father), a athair (his father), and a hathair (her father). Two sample
sentences would be "tá sé mo chara" (he is my friend) and "níl sé a hathair"
(he is not her father).
* for feminine nouns using the definite article "an" (the), and for any
adjectives following that feminine noun. For example, "tá an bhean bhocht
anseo" (the poor woman is here). If you want to say "a poor woman is here"
(as opposed to THE) then you'd have "tá bean bhocht anseo." Notice that the
adjective, bocht, is still lenited even if the feminine noun that precedes
it is not!
* for the vocative case, which is when you're calling someone directly by
name. The vocative case uses an a before a lenited name, as in "A Phádraig,
cad é mar atá tú?" or "A Dhóirín, tar isteach le do thoil!"
* for the genitive case, or for two words in relationship to each other, as
in "bean Pheadar" (Peter's wife) or "fear Mháire" (Mary's husband). You can
also use them with a definite article "an" (the), as in "an phlúr an
gharraí" (the flower of the garden) as long as the lenited word is
masculine.
* for superlatives (very and too). For example, "tá an fear an-fhada" (the
man is very tall), you can see that fada is lenited. And "tá an fear
rófhada" (the man is too tall) the same thing happens but without a hyphen
(unless the word starts with a vowel, like ró-óg, too young). And unlike in
most cases of lenition, for this particular situation and the "genitive
case" above, d, s, and t (in addition to the usual l, n, and r) are not
lenited here.
* there are lots more, but these above are good ones to know.

2. Eclipsis. When a word is eclipsed, the initial consonant is preceded by
another consonant in direct relation to it. So b goes to mb, c goes to gc,
d goes to nd, f goes to bhf, g goes to ng, p goes to bp, and t goes to dt.
In the case of vowels, all vowels are preceded by n-, as in "a n-athair,"
their father. Some letters, like m and n and s and others, eclipse
invisibly, which means you don't see it. Here are the cases in which
eclipsis occurs:
* for the possessive plurals, as in ár gcara (our friend), bhur gcara
(y'all's friend), and a gcara (their friend); or ár bpáiste (our baby), bhur
bpáiste (y'all's baby), and a bpáiste (their baby); or ár sagart (our
priest), bhur sagart (y'all's priest), and a sagart (their priest). Note
that s doesn't show an eclipse!
* for prepositional phrases, as in "ar an mbórd" (on the table), "ag an
ngeata" (at the gate). You'll see this especially in the Connemara songs
you learn in class, but not so much in Ulster Irish (where they lenite
instead!!). T and D are not affected by this, so it'd be "faoin teach"
(under the house), not "faoin dteach."
* for dependent forms of verbs, as in "cá bhfuil" (where is), "an bhfuil"
(is there), etc. You don't just get this in present tense, but in future
tense ("cá mbeidh," "an mbeidh") as well.
* for "genitive case" when two words are in relationship to each other as
plurals. So, the song of the birds would be "amhrán na n-éanacha." (the
eclipsis here is the n- form, remember?)
* there are lots more, but these above are good ones to know.