The Gardaí are currently clamping down on misbehaviour by all road users, including cyclists, so please inform yourself about the specific offences for which cyclists can be fined.
The full list of fixed charge offences for cyclists is outlined below. (Thread)

Lights: You need to have a working front light (white) and back light (red) during lighting-up hours, i.e. at night.
- You do not legally need lights on your bike during the daytime.
- "Hi-vis" clothing is not a legal requirement, day or night.

Lights, continued:
Having no front or back lights is a single offence. Beware of anyone attempting to charge you with two separate offences, i.e. One penalty for no front light, and a separate additional penalty for no back light.

Helmets:
There is no legal requirement to wear a helmet while cycling in Ireland.
You should not be warned/admonished/fined for not wearing a helmet, because it's not illegal to cycle without a helmet.

Headphones/Earphones:
It is not an offence to wear headphones or earphones while cycling.
Anyone claiming this falls under cycling "without reasonable consideration" is not basing that opinion on legislation.

Cycling under the influence:
The legislation governing driving under the influence does not apply to cyclists, but a cyclist can be arrested if a Garda believes they are under the influence to the point where they do not have proper control of the bike.

Finally, please always cycle with care and consideration for others - especially pedestrians.

Any questions?

For more information on cycling laws and your legal rights, please visit @citizensinfo's excellent webpage on the topic here: https://www.citizensinformation.ie/en/travel_and_recreation/cycling/cycling_offences.html