I'm also affected by the issues Three Prepay customers are facing.
Three don't care enough to even communicate that there's an issue (even though it's been going on for about 10 hours at this stage), so my advice would be to order a new sim from https://gomo.ie/
€14.99/month for everything and reviews look good.
Would advise others to do the same, pretty straight forward and you can keep your number.