I think this is another case where it sounds odd to translate "can/can't" into Irish. Some more idiomatic ways might be:

Táim ar bís "I can't wait" "I am in suspense" literally "I am in a vice"
Tá bís orm (same)

To say "I can't wait to (do sth)":

Táim ar bís le rud a dhéanamh
Tá bís orm le rud a dhéanamh

(Corrections welcome.)

Any other ideas?

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Múinteoir Gaeilge - Irish Teacher
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect, but I can also speak Ulster and Munster Irish with native-level pronunciation.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), Gaeilic Uladh (GU), Gaelainn na Mumhan (GM), agus Gaeilge an Chaighdeáin Oifigiúil (CO).

I'd say "tá deifre orm (sin a dhéanamh)"...
"I can't wait" is an English idiom, if you say "cha dtig liom fanacht" in Irish, it means you really can't wait, it doesn't mean you really want to do something.

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

Tá deifir orm means "I am in a hurry". While I suppose it is could be used in the same context as "I can't wait", I think it is used more in cases like "Sorry I can't stay and talk I have an appointment to keep."

Tá bís orm is something like English "on tenterhooks" in the meaning of "in a state of painful suspense or agitated expectancy".

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Múinteoir Gaeilge - Irish Teacher
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect, but I can also speak Ulster and Munster Irish with native-level pronunciation.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), Gaeilic Uladh (GU), Gaelainn na Mumhan (GM), agus Gaeilge an Chaighdeáin Oifigiúil (CO).

Cur chuige eile: "is fada liom go..."

Again context !
Tá sceitimíni orm faoi.... - I'm excited about....

Yeah, I actually split this request in two - it was originally "I'm so excited. I can't wait." - sceitimíní works well.

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Múinteoir Gaeilge - Irish Teacher
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect, but I can also speak Ulster and Munster Irish with native-level pronunciation.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), Gaeilic Uladh (GU), Gaelainn na Mumhan (GM), agus Gaeilge an Chaighdeáin Oifigiúil (CO).

Tá mé ag tnúth/súil/dréim go mór le... might work too in the context.