Freight rates are a closely guarded secret - different rates for different companies depend on volume of traffic -ie how many sailings you use a day / week / month. If its going to be a regular thing probably best going direct to the ferry company if its a one off then some of the big companies on the route you want to use might sell you a spot . Cairnryan to Belfast - try McBurney / Agro / Manfreight . Hollyhead to Dublin - don’t know its not a port we really use .