How would you say "I am having a party today"? Best guess is:

Tá cóisir ag beith agam inniu.

Somehow, that just doesn't feel right to me.

Also, how would I say "I will be having a party tomorrow". Best guess here is:

Béidh cóisir ag beith agam amárach.

Also feels a bit off to me.

As always, any corrections or suggestions will be appreciated.

Although it's written in the present in English I think when written like that it implies the future. If the party hasn't started yet.

I am having a party today.
Beidh cóisir agam. (inniu/ ar ball = today/later) that's if it's your own party.

If going to a friend's party you'd say:
I will be at a party.
Beidh mé ag cóisir.

If already partying:
I am at a party (now)
Tá mé ag cóisir (anois).

"ag beith" is impossible. There's no "I'm having", "I'm being" in Irish
"I'm having" here denotes something from present to future tense,
so: Beidh cóisir agam inniu/amárach (or: Tá cóisir agam anois/inniu), see Bríd's post.

Yeah, ag beith just didn't feel right to me.

Had a feeling it's another one of the cases where a direct translation between English and Irish just doesn't work.

Although Beidh cóisir agam literally means "I will be having a party", I can see how it's the equivalent of "I am having a party".

Thanks all.