cloisim daoine a rá 'is cuma sa tsioc' ach ní fheicim san fhoclóir é.

I've been hearing this phrase around town that's used fairly informally for 'I don't give a frost' or 'I really don't care at all', clearly much stronger than 'is cuma liom'

I can't find it in the dictionary, though. Am I spelling it wrong? Or is it just too new fangled of an expression for the dictionary or just rud éigin a chloistear sa chaint amháin?

Your spelling is right: "Is cuma liom sa tsioc". It could be translated as "i don't give a damn" (i dunno if it's rude or not in Irish).
I've learned it from a teacher at the university. It may be found in some dictionary, i think.

I've heard 'is cuma liom sa tsioc' as well as 'is cuma sa tsioc liom' -- or even just like 'is cuma sa tsioc cad a dúirt sé . . . ' It's doesn't appear to be rude,exactly, just emphatic, but I think it's kind of funny that it resembles the sound, intonation and usage of 'I don't give a 'f' in English.

: )

There's a possibility that Irish speakers who became English speakers simply replaced sioc with f*ck becuase of the similarlity in sound. Just an idea. irish speakers brought a lot of language habits with them into English.