I've only had to get a Tax Clearance Cert twice - the process on Ros was automatic and the cert was issued immediately (maybe my tax affairs are very simple, but I thought it was oddly quick). When that cert was expiring, I got a Revenue notification email to advise of that, and I repeated the same application as I had done first time - also issued immediately.
Having this type of automatic expiry notification should mean remembering to renew isn't an issue.