I doubt cruise and Fota traffic contributes more than 5% of the traffic on the line. I'll grant you the bit about the road; I believe that by a considerable margin Cobh is the largest town not on the national road network.
Really, the line should be more like Midleton, which doesn't have the tourist traffic, is single track and has a dual-carriageway into Cork, but we don't have a baseline for numbers there.
Clearly the biggest difference is that 60,000 people are trying to get into Cork City Centre every day for work or education (there's also a substantial number who go in for shopping, business and entertainment, but these are one off trips); Limerick gets less than half that number. Then again, you could have half Cobh's 24 services a day and still have something that could make people happy.
I'm also serious about incorporating P&R. Near the junction of the old Dublin Road and the current M7 would be a great location.