My buddies at Runnerslevel.ie have provided me with a great blogpost with road races in Ireland to run a 10k pb. Now, while running a personal best should not be your main focus now during marathon training, it's still a great idea to have experience running actual races prior to marathon day. Additionally, these routes outlined in the blogpost serve as great options for those long run days.
As any coach will tell you, you should participate in a few races prior to marathon day only to get a good feeling of the vibe you might expect during race day from fellow racers as well as test your own performance (think what to wear, gel intake, and all the fun stuff that will be useful in any given race) in anticipation of Marathon race day.
Hurry up and read: Road races in ireland to run a 10k pb
Need help deciding what to eat prior to your long run?