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One uses irish moss (i.e., kettle finings=moss, whirlflock, SuperMoss, etc.) in the boil kettle in the last 10-15 minutes. It helps coagulate some proteins out of the wort.
So, the irish moss and all the unwanted proteins attached to it fall to the bottom of the boiler during cooling. Hopefully, one is leaving it behind or straining it out on the way to the fermenter.
If any moss made it to the fermenter, I personally have not read that it negatively affects yeast performance. There are different "finings" used to clarify beers in the final days of conditioning or in the keg. I believe it is possible to use too much of those finings and inadvertantly drop some of the proteins that boost body/mouthfeel, and also drop so much yeast that bottle priming might be tough. Those finings include gelatin and PolyClar.
Irish Moss will not interfere with priming. As ML stated it is used in the kettle, which will not affect the last stage of the process. One of my first AG Batches called for 1 teaspoon of Irish Moss and I added the entire container . No ill effects were noticed and the bottles carbonated with no problem.
Cheers lads. Big help. Still new to this and was not aware i was not supposed to add finings if i am bottling beer. reckon i added a tad too much gelatin to the secondary and my kolsch now seems to be struggling to carbonate