So basically, it'll ultimately depend on the rental company, but there are certainly penalties. It's also definitely possible, so don't worry about that.
For example, from Irish Car Hire Info:
Drivers under 25 must have held their full license for at least 24
months prior to the date of pick up. The minimum age to rent a vehicle
is 21 years (maximum 75). Drivers under 25, i.e. drivers aged 21,22,
23 or 24 are subject to an insurance surcharge of 25 Euro per day and
drivers 70 or over are subject to an insurance surcharge of 25 Euro
per day. All drivers under 25 and over 70 are liable for the first
1,200 Euro of damage to the car (excess). The surcharge is paid when
collecting the car and is not included in the quoted price.
However, there are a few other terms on that page with regards to where you hold your current license, and whether you'll need to supply an international driver's license. But once you've got that, you should be good to go.