Hi Marge,
I went from Astorga to SDC in Sept 2011 and also flew from Dublin.Journey details were as follows :
Flight from Dublin to Madrid - Departed Dublin 6.20am and arrived Madrid Barajas Airport 9.55am (Terminal 2)
I then went to Terminal 4 (via the Airport courtesy shuttle bus) where I got an ALSA bus to Astorga.The slight downside is that the bus to Astorga did not leave the Airport until 1.45pm so I had a few hours to kill however it meant that I did not have to go into Madrid city centre to connect with the bus.The bus went direct to Astorga and arrived at 6pm.Lovely way to travel and cost €32. I was in time to get a bed (almost the last bed I think !!) at the Alb.Siervos de Maria
http://www.caminodesantiagoastorga.com/ but if arriving at this time now you might want to book alternative accomodation (small pension or private Albergue) just to make sure you have a bed for your first night.
ALSA site is :
www.alsa.es
Hope this helps.
Buen Camino
Peadar