If you take the train/bus combo you then have to take a taxi the 3 miles from Lanyon Place station to the port.
Instead you could take the bus X1 at 0645 from Dublin Busaras to Belfast Europa Bus Station arriving 0935 then the Scottish Citylink #923 from Europa to the port at 1020. That takes you to the ferry, then another 923 picks you up at Cairnryan for the ride to Glasgow, arriving at 1625. All for less than the price of a ferry ticket alone.
Or the 0245 bus from Dublin, and as above on the #923 at 0630 arriving at Glasgow at 1225.
Or, as it is a Tuesday, the Hannons bus from Europa at 1030 also runs. That bus drives on and off the ferry with you and your luggage on board and arrives at Glasgow at 1600.
I rarely say this but you may be better flying. This is the early fall when we are starting to get the potential for weather disruption on the ferry route and you have a key appointment to keep.