In this declension, the genitive (possessive) form has the same ending as the basic form. Plurals of the nouns form in several ways, so that you must learn them noun by noun.
Review of changes in the noun in the singular. First masculine nouns:
aoí (ee), an t-aoí, an aoí, na haíonna (HEE-uh-nuh); guest, the guest, of the guest, the guests. The word begins with a vowel, "a" here, so a "t" precedes it in "the guest". Because the noun starts with a vowel, the genitive has no letter prefixed.
Phrases with the word "aoí":
Is aoí í; she is a guest.
D'imigh an t-aoí; the guest left.
Cá bhfuil (vwil) seomra an aoí?; where is the guest's room?
Íosfaidh (EES-hee) na haíonna; the guests will eat.
Ghlan siad seomra na n-aíonna; they cleaned the guest room.
Coiste (KISH-te), an coiste, an choiste (K*ISH-te), na coistí; committee, the committee, of the committee, the committees. In the genitive, the initial "c" becomes aspirated.
Phrases with the word "coiste":
Chuir sé coiste ann; he put a committee there.
Molann siad an coiste; they praised the committee.
D'fhág (daw*g) mé bord an choiste amuigh (ah-MWEE); I left the committee's table outside.
Gheobhaidh (YOH-ee) sé na coistí; he will get the committees.
Ag míniú (MEEN-yoo) na gcoistí; explaining the committees. In the genitive plural, eclipsis occurs after "na", so a "g" sound replaces the "c" sound here.
seic (shek), an seic, an tseic (tek), na seiceanna; check, the check, of the check, the checks (banking term). In the genitive singular, a "t" sound replaces (eclipses) the "s" sound.
Phrases with the word "seic":
Fuair mé seic eile; I got another check.
Scríobhaidh (SHKREEF-hee) sé an seic; he will write the check.
Ag bailiú (BAHL-yoo) an tseic; collecting the check.
Chuir sé na seiceanna sa bhanc; he put the checks in the bank.
Ag milleadh (MIL-uh) na seiceanna; destroying the checks.
If the masculine noun begins with sl, sn, or sr, the genitive singular prefixes a "t" sound after the word "an", meaning "of the". The "t" sound eclipses the sound of the "s". Examples:
sloinne (SLIN-ye), family name; lucht an tsloinne (TLIN-ye), people of the name.
sneachta (SHNAHK*-tuh), snow; dath an tsneachta (TNAHK*-tuh), the snow's color.
sraithchíste (srah-HYEESH-te), layer cake; ag bácáil an tsraithchíste (uh baw*-KAW*-il un trah-HYEESH-te), baking the layer cake.
If the masculine noun begins with sc, sm, or st, however, then the genitive singular stays unchanged, even after "an", of the. Examples:
scála (SKAW*-luh), scale; ar chúl an scála, behind the scale.
smitín (smi-TYEEN), sharp blow, tap; i ndiaidh an smitín sin (in YEE-uhn smi-TYEEN shin), after that sharp blow.
spóca (SPOH-kuh), spoke; ag gearradh an spóca (uh GYAHR-uhn SPOH-kuh), cutting the spoke.
stábla (STAW*B-luh), stable; in aice an stábla (in AK-uhn STAW*B-luh), next to the stable.
Reviewing feminine nouns in the fourth declension:
ola (OH-luh), an ola, na hola, na holaí: oil, the oil, of the oil, the oils. The word begins with a vowel, "o" so an "h" is prefixed to it after "na," of the.
Phrases with the word "ola":
ceannóidh mé ola; I will buy oil.
sheiceáil (hek-AW*-il) sé an ola; he checked the oil.
in aice na hola; next to the oil.
Chonaic (k*uh-NIK) siad na holaí; they saw the oils.
Ag meascadh (MASK-uh) na n-olaí; mixing the oils.
bá (baw*), an bhá (vwaw*), na bá, na bánna; bay, the bay, of the bay, the bays. Aspiration of the initial "b" occurs in the basic form after "an", the, but not in the genitive singular.
Phrases with the word "bá".
feicim (FEK-im) bá; I see a bay.
is í sin an bhá; that's the bay.
Ar thaobh (HAYV) eile na bá; on the other side of the bay.
Bhí na bánna tanaí (TAH-nee); the bays were shallow.
Os comhair (KOH-ir) na mbánna (MAW*-nuh); in front of the bays.
slí (shlee), an tslí (tlee), na slí, na slite (SHLI-te); way (road), the way, of the way, the ways.
Phrases with the word "slí":
Fuaireamar slí; we found a way.
Tar an tslí seo; come this way.
Ag fáil na slí; finding the way.
Tá na slite dorcha (DUHR-uh-huh); the roads are dark.
Ag foghlaim na slite; learning the roads.
Feminine nouns beginning with sn or sr also have a "t" that eclipses the initial "s" in the basic form:
sní (shnee), an tsní (tuh-NEE), na sní, (no plural); flow or pouring.
sruthlíne (sru-LEEN-e), an tsruthlíne (tru-LEEN-e), na sruthlíne, na sruthlínte; streamline, etc.
Feminine nouns beginning with sc, sm, or st do not undergo this eclipsis:
scige, an scige (SHKIG-e); mockery, the mockery.
smearaithne, an smearaithne (smar-AN-uh); slight acquaintance, etc.
spleá, an spleá (splaw*); dependence, the dependence.
státchairde, an státchairde (staw*t-K*AHRD-e); moratorium, etc.