Grant (Silentjewel),we had a great day out in Ireland,done the Antrim Coastal Route ,same as yourself ,then a high speed blast back for the ferry.
Very dull ,wet/damp roads when we got there, but it turned out a really sunny and warm day
Same as you ,we got ferry with 5 mins to spare , infact when leaving Portrush it was "every man for themselves",riding at stupid speeds the 60 miles or so south ,to get the ferry, we kinda got split up into 3 small groups , i think we all thought we were going to miss it
Thankfully we never saw any speed cameras anywhere , or even a single police car .
We really didnt have enough time ,the coastal route is very scenic ,but i think we stopped twice for 10 mins max, and had 10 mins in Portrush ,good job we got fed on the ferry
The thing is,after the blast to get the return ferry ,at the Port check-in the woman said (in Oyirish accent)"those are flexi-tickets,you couldve returned by the 8pm ferry
What a bargain at £20 return ,the food was excellent ,a smashing big bacon bap(roll),and tea or coffee on the way there ,and main meal on the return journey.
I opted for the fish and chips and mushy peas ,which was supposed to sell for £8.50,and they even let us have a coke instead of the tea or coffee.
We met "Blackneck" (forum member) from Stranraer , at the outward ferry terminal ,who came and sat with our group for most of the ferry journey going there ,then went his own way to meet someone in Antrim,... nice to meet you Edd

I'll give you a shout next time were headin up the west coast
The strangest thing was us getting overtaken on the coastal route by 6 bikes ,including one with a small sports sidecar thingy

,...mind you it was a GSX1400 ,it sounded mental ,....and the guy ridin it certainly was
All in all a great day out ,...and i see there still selling the same £20 cheap deal for the Ulster GP..
http://www.stenaline.co.uk/ferry/day-trips/bikeday/