To use this, you will need the Aer Lingus PNR, which is of the format 2XXXXX. This may already have been shared with you by your agent, though it sounds like they're the less-helpful kind of agent who doesn't pro-actively share this kind of information and who probably makes it difficult and/or expensive to contact them. If you don't have the airline PNR, you would have to contact the agent to find out what the airline PNR is.
I have the PNR and can bring it up on aerlingus.com. But under baggage is just blank space, no details. The OTA displays 1x 20KG bag.
iii) if you have such difficulty with EI tickets bought from this agent - you refer to "massive issues" and having to claim back bag fees - perhaps it's time to consider buying travel directly from Aer Lingus, or from some other agent.
First time booking EI flights through this agent. The massive issues I've had are with bookings made directly with Aer Lingus and via Avios bookings. Granted they are always from France - sometimes small issues in Dublin but they always sort it out in Dublin without charging me.
3 of the last 4 times I had to pay and do a chargeback in France. The last time was in CDG, it was a standard booking online and paid for the 20KG check in bag. I showed the email receipt and confirmation showing the baggage. The agent refused to take it even though the check in machine let me print out the bag tag without issue. Finally they accepted it when I showed them the twitter message.