Raglan Road does open at 3pm every day.
There are three bars at Raglan Road, two large ones inside and one nice one outside. There is no problem just going in and sitting at the bar. You can also eat at the bar if you want to, no problem there either. If you do want to sit at a table though, you will need a reservation or have a short wait.
Raglan Road does seem to get busier as the night goes on. Enjoy!