Sounds like you are in very good condition, so this shouldn't be too difficult.

How hilly is the ride? Is road condition generally good?

My recommendation would be something fast and comfy. I would skip the full fendered tanks that many here use. Those go anywhere no matter the weather bikes are great when your ride is 8 miles and you do it in all conditions.

You have different needs. You want to go a looonnnnng way and you can pick and choose days. I am certain that even in Ireland, there are days when the weather is decent

You should be able to find a good quality used steel roadie. Maybe aluminum if you get lucky, but, I am not sure that you'd gain much.

Look for the lighter aluminum bike if you have a hilly ride. Steel may be better if there aren't many hills as they tend to ride nicer. And make sure you've got a very comfy saddle.

For accessories, a PB superflash rear blinkie or two would be nice. A seat bag should provide all the cargo carrying space you need. Skip the full rack. Remember, you wanna stay as light and fast as possible.

I would also recommend you have a look in the ebike forum. One of their selling points is that they can greatly extend commuting range. There are some there that do commutes in excess of 20 miles on a daily basis.

One other thing. IMO aero bars are a must. Not so much that they make you faster as they make you more comfortable. My aerobars are a godsend for my wrists and back.

Good luck.