Can this mean "I'm eating stew and pork" as in English, or only that I have them in my possession?
No, that would be Táim ag ithe x (I'm eating x).
Tá x agam means I have x, not I'm having x.
I don't know about irish, but Scottish Gaelic would use the verb "to take" in this sense, just like most other Western European languages do.
"pork stew" is counted wrong. Pork stew seems more natural than separate stew and pork, but apparently Irish doesn't use this construction for that.
“Pork stew” would be stobhach muiceola, using the genitive of muiceoil (i.e. “stew of pork”).