Don't get too concerned about flight plans. Ask National Flight Centre to show you. They are very simple to do, and you aren't stuck to the route that you put in there.
You need a flight plan for any entry into controlled airspace in Ireland. Weston is inside controlled airspace, hence why you would need it.
In practice Weston will let you in and out without a flight plan. So if you are really concerned about this, you can operate without it.
For Inishmore you do need to get PPR first. You can get it from Aer Arann. Their contact details are here
Aer Arann Islands - Contact
Things have changed a bit since Mark 1 was there unfortunately. You can now only land or take off while the airport is maned and there is a landing fee of €10.
On a seperate note, I'd call National Flight Centre to make sure your booking is still in the system before traveling to Weston. They used to (can't speak about recent years) have a reputation for cancelling booking to suit themselves and not notifiying the pilot until they arrived at the airport!