Which airport should I use to get back to Dublin from St Pancras?

The closest airports from central London are Heathrow and the tiny London City Airport. But Luton Airport (30 miles away), which a lot of discount airlines use, is served by rail at Kings Cross station which is right next to St Pancras. Use ths site to find lfights: www.whichbudget.com
Off hand I believe Ryan Air flies between Luton and Dublin.

As Michael says, Ryanair flies from Luton to Dublin. However, as of December the King's Cross Thameslink station closed and trains to Luton now depart from St. Pancras Domestic. Take the train to the Luton Airport Parkway station and the free shuttle bus from there to the airport. Use the Journey Planner at www.nationalrail.co.uk to get timetables and fares.

Your best choice might be flying from Heathrow to Dublin on AerLingus because there are many flights each day. You can take the Tube directly from St. Pancras to Heathrow on the Piccadilly line in just under an hour for £4.

Are there any good flights from London City Airport (LCY)? It is a small airport, but it is always my first choice for intra-European flights from London. It is close to the city, easily reached by the DLR (tube connects to DLR) and check-in and security are a breeze.

However, did you check to see if you can fly directly from Paris to Dublin? It might be cheaper and save you time.

Thank you for your suggestions, they were very helpful. I did go to the whichbudget.com website to find which budget airlines flew where and then got on the incredibly helpful nationrail.co.uk web site's journey planner. That showed the time it takes and how simple/complicated it is to get all over London from anywhere else in London. It seems that the best bet is to use Ryan Air ($111 round trip from Dublin to London in July!) and go out from Gatwick. It is a little farther but actually simpler and quicker because of the single train - first capital connect. Luton was another option but there are much later flights out of Gatwick which means I can spend more time in Paris before taking the Chunnel back. Thank you again!

Its actually worth checking out Aer Lingus and BMI direct from Heathrow on their respective web sites.
My wife is Irish and goes to Dublin several times a year and often they're cheaper if you book well in advance.
They are not budget airlines but they are often cheaper than Ryanair whhen you factor in all the extras of Ryanair.
Ryanair charges extra for checking in, for each item of baggage etc. BMI for instance has a full baggage allowance and there are none of these extras. Also its only 4 GBP direct from St Pancras to Heathrow on the London Underground, much cheaper than Gatwick or Luton.

Laura, Thank you for your advice. I had not thought of flying directly back to Dublin from Paris - I assumed I had to do a round trip between Dublin and London and had thought of Paris as a 2 day side trip. But, I had not bought any tickets yet so I looked into flying from Dublin to London, taking the chunnel to Paris, and flying from Paris back to Dublin and you were so right. It is both easier and cheaper and I get to spend a few more hours in Paris. Perfect. Thank you!!