Delighted you're happy with the info, and to get the certs just apply to www.groireland.ie
. Fill in the details including volume & page nos (ignore the question about PRSI no, which is an Irish tax no.), give them your credit card details, and the certs will be posted, and should arrive within a few days. Hope you get some useful info from them!
I had a look on ancestry.com for marriages of Patrick/John/Jack Keegan c. 1925-40, but there are too many to figure out which is the right one (although there was only one for Jack Keegan, who married either Emily Boland, Charlotte Chambers, Bertha Cohen or Margretta McConaghy). It therefore might be worth your while to follow up on Ursula's info re Peter/Peadar Keegan who joined the RAF in the 1950's, as this seems to be at least one definite half-sibling of yours.
I don't know if you have your father's baptism cert, but in the recent past I had a huge stroke of luck when a note on a baptism cert I acquired told me who the male member of my family had married, and when - this was because he married outside his Parish. Might be worth a shot!
Best of luck!