I've no experience with joint agencies, but I doubt any agency will work without the prospect of earning fees.
Even using a single agent, if someone approaches you directly and you arrange the sale yourself without their help, their fees are still due. In fact in most cases, the fees are still due for a period like 6 months if you sell after terminating the contract with the agency.
Looking around Dublin, I've never seen joint agents being used, but perhaps in a more remote area with fewer prospective buyers, or a more unique property it might have advantages. With most agents advertising pretty much everything on sale via Daft & MyHome, more than one agent may not attract much more interest.