I've only bought one bag of Irish Moss in at least 2 years. It's so cheap but is there really any need to replace it?
Irish moss is a dried, dead product (seaweed), which if kept in a air-tight jar out of sunlight will last pretty much indefinitely. I'm using mine that I bought 4 years ago and the wort still clears nicely.
Some other anecdotal evidence, david_42, senior member on HBT writes
So I think you're fine with 2 year old moss.