Because there are some very cheap options on low cost airlines, I would suggest flying as a convenient alternative.
Ryanair and Aer Lingus have various flights each day.
travel.stackexchange.com/users/1381/Rory Alsop
If your preferred option does not work out, you could always try Eurolines Coaches, which depart to many European Countries including Ireland from Victoria Coach Station, London.
More info: http://www.eurolines.co.uk/destinations/ireland
travel.stackexchange.com/users/3692/Simon
Ferry bookings work in the same way as flight tickets these days, the actual fare depends on the date and time of the booking, and changes in time.
To check the fare that is of interest you need to go to the ferry company website, e.g. Stena Line , enter the dates, press continue booking, enter the number of passengers and the size of the car, and then you will have all possible ferry times with the combined fare---which you can book.
Unfortunately you will have to do similar steps to check the prices of all the other means of transport.
If I have to guess, I would say that the ferry only makes sense if you already own your vehicle.
Otherwise, it will make more sense to get to Dublin on the public transport, and rent your car there.
travel.stackexchange.com/users/44/Grzenio
If you'd prefer to get a train, the cheapest option is generally a "Rail & Sail" ticket: http://www.stenaline.co.uk/ferry/rail-and-sail/
In fact, it's usually cheaper than paying for a car on the ferry (and that's excluding the cost of fuel to get from London to Holyhead).
As well as being cheaper, you also have the peace of mind that if there are any delays, you're still covered for the rest of your journey.
travel.stackexchange.com/users/4999/lucas
Rome2Rio gives you the alternatives.
Rome2rio gives airfares, but not for the ferry though.
You can get a detailed quote from Stena line
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London to Dublin: bus, train, plane, or car rental? I would like to travel from London to Dublin (with family). But the web-sites for the ferries seem to be vague about the cost for automobiles. (And they seem to be focused on the last-minute travelers).   The date is May, 2013. I would prefer the train/ferry, but what are the other options?
London to Dublin: bus, train, plane, or car rental?
travel.stackexchange.com
<trains><transportation><london><ferries><dublin>
travel.stackexchange.com/questions/13985
travel.stackexchange.com/users/4647/Joseph Quinsey
Low cost airlines such as Ryanair and Aer Lingus have some very cheap options and are a convenient alternative, but you can also try Eurolines Coaches, which travels to many European countries, including Ireland, from Victoria Coach Station, London. Ferry bookings can be made on the website as with flights, and the fare depends on the date and time of the booking, as well as changes in time. The cheapest option for trains is generally a "Rail & Sail" ticket. You can find the alternatives on Rome2Rio, but it will not give you the ferry options. There are a number of ways of travelling from London to Dublin. Flying is a cheap option and there are daily flights. Ferry is available but the booking process is a little complicated. Rail and Sail is another option. Moreover, Eurolines runs a bus service out of Victoria Coach Station. Alternatives can be found on Rome2Rio but it does not have the ferry fares.
Flying is a cheap way of travelling from London to Dublin. Ryanair and Aer Lingus have daily flights. Eurolines coaches leave from London Victoria Coach Station. Ferry booking is complicated as you have to enter all your details to get a quote. A better option may be Rail and Sail which means you can travel without your car. Rome2Rio give all possibilities but not all the fares.