Good morning to you, certainly have your hands full there!! I don't have any direct contacts for Northern Ireland but would suggest you try this link for the DSA group there https://www.downs-syndrome.org.uk/northern-ireland/
Would also just like to mention that I was lucky enough to go on a weekend away with young adults with Downs syndrome a few years ago and can only say that the youngsters from Ireland were fantastic fun and had the time of their lives.
Hope this helps. My daughter is 26 years old, one of twins and can do almost everything that anyone of 26 can do. The only bit of advice that I can give you, as an experienced Mum, is don't lower expectations or make excuses for her, your other children will probably not appreciate it!