transport questions

Hello, I will be coming to Europe in June and my route is Madrid - Barcelona - Paris - London - Dublin. My questions are:

For Madrid to Barcelona, what is my best/cheapest option? Would I be able to book a train or bus for a reasonable price (or at all) when I arrive in Madrid for a few days later?

I found a flight from Girona airport in Barcelona early in the morning to Beauvais airport in Paris. My only concern with this is the price to get to/from these airports from the city centres. Does anyone know airport transfer options for these airports, and the times they run?

Paris to London. Any suggestions on what mode of transport and how far in advance I would need to book?

ryanair flies cheaply from London to Dublin from Stansted, Luton, and Gatwick airports. Which would be the cheapest airport to get to if I'm staying near Oxford street?

To anyone who takes the time to answer these questions... thank you very much.

Josh

For all routes book as soon as you can with ryanair or easyjet as the sooner you book the cheaper it is. Do not wait and try to get a last minute flight as there are now no more last minute cheap options. I think all airports do a bus service to Victoria in London and from there you will have to get another bus or the tube.

I am not sure what the cost difference is, but if you don't have too much luggage to carry on a train, I would take the Chunnel Train from Paris to London. If I remember correctly, it is 2 to 3 hours, and you get to see some beautiful country side as you travel.

From city centre to city centre, London St Pancras to Paris Gare du Nord by Eurostar is a better option than flying, however ticket prices are expensive, so if there are any bargains to be had, you will need to book well in advance.

London Euston to Dublin via Holyhead can be done by train and ferry, but at a very expensive cost. Another lengthy, but far cheaper, alternative to the train is the Eurolines bus from London Victoria coach station.