san is the form of sa used before a vowel. Similarly, without an article, you use i(n), adding the "n" before a vowel:
i gcuisneoir: in a fridge
in Éirinn: in Ireland
sa chuisneoir: in the fridge
san oíche: in the night
sna Gardaí: in the police force
All the gory details here: http://www.nualeargais.ie/gnag/i.htm
You'll run into this a lot more with other prepositions. "Ta/ ochras orm." = "I'm hungry." Literally, "Hunger is on me." You wear emotions in Irish, so, "Ta/ bro/n orm." = "I'm sorry" because it literally says, "Sorrow is on me." Ta/ isn't exactly "is." It's literally "stands." But "Is" as in "Is cat me/" (I'm a cat.) is the "is the same thing as" kind of "is." Ta/ is the "stands as" kind of "is." (Sorry I've had to put the accent marks after the vowel above, hope you catch my meaning.)
Tá = is. Because Irish doesn't have a verb for "to have," they express the idea by saying "(possessed thing) is at (possessor)."
So, for example, if I wanted to say "I have a dog" in Irish, I would say "Tá madra agam." Which literally means "A dog is at me" (Tá = is, madra = a dog, agam = at me).