Ireland, Wales and #irewal have all soared to top terms in Twitter trends.
Here's some of the immediate reaction to the epic match from the British Isles:
@holyschmoke (Dublin, Ireland): @garthicus #irewal #6nations #rugby The nation will party tonight - it'll be Paddy's day all over but better!
@paulbuk (Letchworth, U.K.): Ireland won the grand slam after one of the most exciting games if rugby I have seen for a long time
@jonathangrimes (Skerries, Co. Fingal, Ireland): A generation of young rugby players have been truly inspired #irewal
@kevin_brown (Kent, U.K.): Wow. Exciting rugby match from Cardiff. Well done Ireland.
@dragozov (Laytown, Ireland): Congrats Ireland!I'm not a rugby fan, but know how much it means for everyone. #irewal