As do many other European languages, Irish distinguishes between feminine and masculine nouns. This is what you need to know about that for starters.
Nouns are names for things and concepts.
The feminine vs. masculine (gender) distinction is above all a grammatical one. It is related to the natural distinction between sexes, but only related. For instance, the word cailín “girl” is grammatically masculine, while the word stail “stallion” is grammatically feminine.
The most important thing to know at this stage is, that the definite article an (na in plural) affects the first letter of the noun in different ways, depending on the gender.
If the masculine noun begins with a consonant, the definite article does not affect it in the nominative form: fear ‘(a) man’, an fear ‘the man’.
If the masculine noun begins with a vowel, the definite article adds a t- to the vowel: aer ‘air’, an t-aer ‘the air’.
If the feminine noun begins with a consonant, the main rule is that the definite article lenites it, which is seen in writing as a -h- added after the consonant.
b > bh (pronounced w before a, o, u/v’ before e, i): bean ‘(a) woman’, an bhean ‘the woman’
c > ch (pronounced like the German ach sound before a, o, u/like the German ich sound before e, i): cnámh ‘(a) bone’, an chnámh ‘the bone’
f > fh (dropped entirely in pronunciation): flaithiúlacht ‘generosity’, an fhlaithiúlacht ‘the generosity’ (pronounced as “a’ laithiúlacht”)
g > gh (pronounced like the voiced equivalent of the German ach sound – something like a French r – before a, o, u or a consonant, but like a “y” sound before e, i): gaoth ‘wind’, an ghaoth ‘the wind’, girseach ‘girl’, an ghirseach ‘the girl’. Note that a consonant between the initial gh- and the vowel kind of protects it so that an ghrian ‘the sun’ has a “French r” kind of gh before the -r-.
m > mh (pronounced like bh above, but can give the following vowel a nasal quality)
p > ph (pronounced as f)
D and t can be lenited under other circumstances, but after the article an, they never are.
S can be lenited under other circumstances, but after the feminine article an it instead becomes t-. This is shown by writing both the original pronounciation (s) and the actual one (t). Thus, sráid ‘street’, but an tsráid ‘the street’, the latter pronounced exactly as if it was written an tráid.
If the feminine noun begins with a vowel, it is not affected in any way.
Let’s look at some examples:
bean woman, an bhean the woman (feminine)
fear man, an fear the man (masculine)
bille bill, an bille the bill (masculine)
fréamh root, an fhréamh the root (feminine. Note that this has a very common parallel form: préamh, an phréamh)
guth voice, an guth the voice (masculine)
grá love, an grá the love (masculine; but note that the other word for love, searc, is feminine: an tsearc)
girseach girl, an ghirseach the girl
cailín girl, an cailín the girl (as you see, this is grammatically masculine, while girseach is grammatically feminine)
cnámh bone, an chnámh the bone
méara mayor, an méara the mayor
maighdean maiden, an mhaighdean the maiden
The nouns ending in -óg, -eog are feminine:
fuinneog window, an fhuinneog the window
maróg pudding, an mharóg the pudding
The nouns ending in -ach can be both:
Éireannach ‘Irishman, Irish person’, an tÉireannach ‘the Irishman, the Irish person’
baintreach ‘widow’, an bhaintreach ‘the widow’