Hi!
Congratulations on the birth of your baby!
I am also in Northern Ireland. My third son, who will be 6 next month (where does the time go?!) was born with Downs. Like you, we didn't know until he was born. In fact, one doctor had examined him and said we could go home without realising!
It was a shock at first, but maybe made easier as I already had two 'healthy' children. My son did need heart surgery at 6 months, but other than that and having an almost constant snotty nose all winter, is very healthy.
I think maybe you are right to hold off getting involved with the DSA for now in a way. Maybe until you have come to terms with thinsg more. I can remember just thinking of my son as my baby, the Downs didn't really come into it. I found it awkward around other people's children who were older and had Downs at first and didn't relate my son to them. Now I find it much easier, and my son is really just my son and I can honestly say I feel no differently about him to how I feel about my older two sons. In fact, I am pregnant again with twins, and my husband and I both agree that we would not mind one bit if our children were born with Downs, and so have foregone any tests offered, apart from scans.
I am not amember of any DSA over here. I had my son when I was still in England. If you want to, I think you can email me via my profile. If not, give me a shout on here and I willlet you have my email address if you want it. If I can help in any way, let me know. I was very lucky to meet up with someone the same age as me (27) when I had my son, and her baby girl, born with Downs, was only a few weeks different in age to my son. It made a huge difference having a friend in the same situation, and had we met any other way, we would have still been friends.