Eclipsis (griech.:omission, disappearance) is the substitution of the unvoiced consonants with the corresponding voiced ones (c becomes g, f becomes bh, t becomes d, p becomes b).
Voiced consonants are then replaced with nasals (b becomes m, d becomes n, g becomes ng). This is why eclipsis is sometimes called nasalisation.

Here is some background about the causes of eclipsis.

about the written notation of eclipsis:

• In written Irish, the consonant replacement appears in addition to the consonant being replaces, the latter disappears in speech ("ng" is only the velar nasal without the g , as in the German: "lang")
• in titles and similar capitalisations, the eclipsing letter appears in lowercase, the eclipsed one in uppercase: mB, gC, nD, bhF, nG, etc. e.g.: i gConnachta = in Connacht, i nGaeilge = in Irish, i mBéarla = in English
• preceding a vowel one inserts an n- (see n-prefix)

Besonderheiten:

Use:

1. after the prepositions ag, ar, as, le, mar, ó, faoi, roimh, thrí, thar, um if with the article an: (in Munster always, in Connacht by d, t no eclipsis as well as a t-prefix preceding s by femin. nouns) e.g.:  ar an mbord = on the table
2. after a preposition with an article sa (in Connacht always, in Munster only by f, in the standard/Ulster, lenition):
   e.g.: sa bhfarraige = in the sea
3. after the preposition i  if without article  e.g. i dteach = in a house
   (similarly also after the archaic prepositions go = with, iar = to )
4. after the genitive plural article na  e.g. na dtithe = of the houses
5. after the plural possessive pronouns   ár, bhur, a e.g. ár dteach = our house, a gcarr = their car
6. after the numbers 7-10, (17-19, 27-29 etc.) e.g.: deich dteach = ten houses
7. verb after verbal particles / conjunctions   an, go, nach, mura, dá, sula, ach a
8. verb after the interrogative cá (in the meaning "where")
9. verb after the indirect relative particle a
10. verb after the generealising relative pronoun a ("all, that ")
    as well as after the expression diabhal a ( = ní,  "not", lit.: "devil all, that ")
11. after the prefix éa- ( = "un-") Here, one leaves out the sound being eclipsed (éa- + cóir = éagóir = injustice, éa + trom = éadrom = not heavy, light )
12. in a few place names after those nouns which are normally things, e.g. Loch nEachach (Lough Neagh), as well as after other nouns in the genitive plural, e.g. Mainistir Ó dTorna (Abbeydorney), lit.: "Abbey of the O'Dorneys"