Hi Dave,
I've travelled over for several years either using Holyhead-Dublin crossing or flying from Stansted, probably repeating what has already been said but here's my observations,
1) Wed(night) -Wed was always a cheaper option/offer.
2) Booking accommodation through one of the holiday cottage companies will also include a ferry crossing, which is cheaper than booking separatley
3) Booking online can get you a discount
4) Flying, If you can get a cheap flight ( almost certain ), then hire a car at the airport will probably cost you the same as the ferry. You will save time the only problems are that you will be restricted in the amount of tackle you can take. Ryanair for instance now charge for rods going through fragile and have reduced the baggage allowance, getting a inflattable life jacket through check-in and then security even with clearance from a supervisor can turn into a debacle ( allow 60-90 mins for this !! )
Good Luck