Special Olympics Ireland are doing great work. I really admire the organisation since I did a small bit of work with them around the World Games in 2003, and since then have always kept an eye on what they do. I really like what they are starting to do online and on Social Networks.
They recently launched this nice Send Team Wishes on their Facebook page. They also have a national collection day on April 15th (I heard that they raised 1 million last year from it, not sure how true that is). What I like about it is that its easy for me to sign up to volunteer my time. Check out the link here.