When the closest vowel to an 's' is an 'e' or an 'i', the 's' gets pronounced like 'sh'. If I remember correctly and understand properly, Irish divides its vowels into two categories: broad (a, o, u) and slender (i, e). How you pronounce a consonant is affected by whether the closest vowels to it are broad or slender, and no mixing is allowed -- the closest vowels before and after a consonant have to be either both broad or both slender. That's where the silent 'i' in 'páiste' comes from: it's there to balance the slender 'e' on the other side of the 'st'. :-)
(If I've misremembered or misunderstood, I certainly welcome corrections!)
That is correct, but there's a simpler way to see it (although the length of this comment might not seem that way!): Irish has broad and slender pronunciations for all its consonants - normally differentiated by a small "y"-like sound with the consonant, or curling the tip of the tongue up. Some consonants completely change, like s~"sh" and dh/gh~"y"; th is the only one that doesn't change.
Because of this, cad and cead are not said the same - they're respectively (roughly) "kad" and "kyad".
Normally e and i are around slender consonants, so except at the beginning or end of a word, they are used to indicate slender consonants: always preceded by i and followed by i or e, which is why they're called the slender vowels.
So a, e, i, o, u after slender consonants are ea, e, i, eo, iu, and ai, ei, i, o, úi before.
OTOH they are a, ae, ui (and ao), o, u after a broad consonant, and a, éa, io, o, u before.
Before ae, ui and ao, broad consonants take on a slight "w" sound (without rounding the lips if you can), which is why Gaeilge, duit and daoibh sort of sound like "gwehlgya" "dwit" and "dweev".
A lot of words' pronunciations differ only in whether their consonants are broad - and many change their endings to slender to become plural: like capall - "kapal" to capaill - "kapalʸ".
- Tá capall agam - I have a horse
- Tá bó agat - You have a cow
- Tá gabhar aige - He has a goat
- Tá caora aici - She has a sheep
- Tá tarbh againn - We have a bull
- Tá asal agaibh - You (pl) have a donkey
- Tá cearc acu - They have a hen
To have - a bheith ag ...
For example: Ní mór duit airgead a bheith agat chun earraí a cheannach - You need to have money in order to buy goods.
Tá isn't "I" here. Tá is a verb.
The Irish for "X has Y" is Tá Y ag X. When X is a pronoun ("I", "he", "you", etc), it combines with ag - when the pronoun is "I", the prepositional pronoun is agam.
Tá leabhar agam - "I have a book"
Tá leabhar agat - "You have a book"
Tá leabhar aige - "He has a book"
Tá leabhar aici - "She has a book"
Tá leabhar againn - "We have a book"
Tá leabhar agaibh - "They have a book"
Tá leabhar acu - "They have a book"
Tá leabhar ag Pól - "Paul has a book"
Tá leabhar ag an bhfear - "The man has a book"
Tá leabhar ag mo chara - "My friend has a book"
Tá is the present tense form of the Irish verb bí, which is equivalent of "be" in English. In other words tá means "am", "are" or "is", depending on context (the English verb "be" is very irregular).
But Irish doesn't have a verb that means "have". Instead Irish uses the construction tá ... ag ... (in the present tense). To say "X has Y" in Irish you say Tá Y ag X.