Howya John,
To help, If you join SAI-SAC, for your membership fees you get full IFSA affiliation which means you can fish any open competition in the country. Also included in that is insurance, handy if you send a lead winging through somebodies windscreen etc.... :D On the club side of things, you then are able to compete in both our Boat and Shore Master angler competitions, winners of each go off to fish the Master Angler the following year from which the Irish team is selected.
The competition fixture list has yet to be announced, however they will be spread all over the country. You do not need to attend every one to mount a good charge at the master angler title there will be a couple of "spares"!
IFSA numbers are just a number thats allocated to you when you join an IFSA affiliated club. The other Membership numbers you may have seen belong to the SACN, the Sea Anglers Conservation Network
http://www.sacn-online.org and are members ways as such of publicising, quietly, this important area.
I can highly recommend you join SAC, this is not an old fuddy duddys club, we have a good time at the comps and they usually go hand in hand with a few pints and a bit of craic. We fish a variety of venues in totally different areas which can be a great leveller on the day. There are some super Anglers in the club, indeed, MB.3 came in the top ten in this years Master angler comp. You will enjoy the atmosphere and you will learn a lot. It was the first club I ever joined and I was pleasantly surprised at the friendly vibe throughout.