I rented a hybrid car for next Apr 29th (DUB) for three weeks for €232 all in (via Costco with Budget), using Costco Visa insurance coverage. No deposit and free extra driver. I also rented another car for ONE night in the US for $56 (to get home from the airport at midnight). I moved my entire trip because the rate tripled for April 30th. I can't explain the pricing anomaly.
Usually, the deposit is just a hold on your credit card for that amount and it only applies if you don't opt in for the various insurance coverages (liability, collision, tires/wheels, glass). The overall experience of car rental in Ireland can be a trial as the agents appear to be paid commissions on the insurance items. All the insurance will likely double you daily rate. If you call your card company, they'll raise your limit if you have at least a fair payment history.
If you have a credit card that will cover Ireland rentals (only some do) bring proof with you to show the agent. The card companies fill requests for proof routinely.