Many of these groups have information about meetups and shops in the areas. Yes the categories are a little loose and I've probably got things wrong in some instances, feel free to comment and I will correct as I can. I haven't sorted these in any real order and the only Irish group I didn't include are groups that were for an event that has passed. There is a second post coming shortly about shops/businesses
One of the biggest ones is the Irish Knitters Group they have some exellent resources there, in the pages tab you will find Irish Knitting Charities; Irish Knitting Groups and Irish Knitting Shops which also has a link to the Google Knit Map of Ireland.It is also home to the infamous End of Aran Knitters Thread that tries to dispel some of the myths about Aran knitting, it also includes some discussion of current Irish Knitting. Please be aware that many of the "local" posters are pretty tired of this thread and will pounce on people who make sweeping statements that include "I didn't have time to read all the posts on the thread but..."
Irish Spinners is for Irish Spinners of all levels
Crocheters in and around Ireland like it says, for Irish and Irish orientated Crocheters.
If you're looking to practice Irish and knit (chniotáil) or Crochet (chróise) in Irish, you might want to try the Gaeilgeoirí ar Ravelry group
The Dublin Knit Collective Group is often an expansion of the Dublin Knit Collective Blog also posts on Twitter; this is a good one-stop shop for whats happening in Dublin and various posts about the Sock challenge and Dublin happenings.
Shebeens is a group for the South-West
Rainbow Stitch 'n'Bitch is based in Limerick
Cork Stitch & Bitch is based in Cork
Wicklow Knitters are based in Wicklow
Cafe Create are based in Galway
Norn Iron Knitters are based in Northern Ireland
Belfast Stitch & Bitch a group in Belfast, Northern Ireland
The No 1 Ladies Knitting Group are based in Wexford
Smaller meetup/activity groups
Fiber Fun Friday is the group for the fortnightly meetup in the Tea Garden in Dublin
Chronicles of Yarnia an extension of the Witty Knitters who meet in Celbridge Co. Kildare they have a blog
Lisburn stitch-n-bitch is a group that meets in Lisburn, Northern Ireland
Innocent Smoothie Big Knit Group
St Patrick's Day Celebration note calling it St Patty's in this group will lead to online mutterings and high blood pressure among Irish folks, please avoid.
Irish Charity Groups
Utando are a group who make handmade dolls for the children of KwaZulu-Natal, whose lives have been affected by HIV/AIDS
Isands group a group who create burial wraps and gowns for early loss babies
Other Irish Groups or Irish founded groups that have an international slant
The Irish Book Group
Irish Hyperbolic Crochet Reef this started as a single event and seems to have taken on a life of it's own. Blogging here
Labradoodle Knitters founded by an Irish person it shows up when you search for Irish groups in Ravelry
Fans of Emmerdale another Irish founded group
Magic Mums group