This is Ryanair; they stick rigidly to the rules.

Don't expect them to accept your bag until the advised baggage acceptance times - even though, as in DUB, the desks will be manned all day.

Unfortunately, there are no luggage storage facilities at the airport. (The Excess Baggage company does store luggage, but for a minimum of 3 days. So whether they would take your bag for a few hours, they are probably going to charge you for 3 days).

Don't forget that Aer Lingus also flies to GLA and they would almost certainly allow you to check your bag in "early".