It's quite far. I had a MSC cruise last year from Trieste and the coach transfer took an hour and a half with fairly light traffic. I'd think a taxi would be fairly expensive. it might not be that far in km, but it is a narrow winding road for a lot of the wa.
The Ryanair web site provides airport transfer information to Trieste as follows
Trieste Ronchi Airport is situated 33 Km outside of Trieste city. There is a bus connection to the city centre leaving from outside the Terminal building. This is operated by APT Gorizia; the journey time is 50 minutes and tickets cost €2.85. Timetables and any additional info can be found on www.aptgorizia.it.
You may wish to consider getting the bus to the city centre and then a taxi to the cruise terminal from there.