Those fares are correct. Sunday is defined as a peak day for intercity services (a family return on Saturday would be €74).
There is indeed a promotional fare of €15 from Longford to Dublin on weekends (see
http://www.irishrail.ie/your_ticket/fares_enquiries.asp near the bottom). I believe car parking at Longford is €2 a day.
You will not be able to have five people sitting together in any case (you changed from four to five halfway through your post) as there are no banks of more than four seats together.