I'm sure people have done the marathon with a training background like that when it wasn't their first, and they had (in the longer term) the training base and experience to sort of muddle through.
But your first? It might be a thought to look for a marathon you really like the sound of, next spring (or even at the turn of the year if you can afford it), book in for that, and then just have a nice weekend in Dublin but focus on recovery.
Alternatively you could follow the physio's advice for getting to the start line *healthy*, crosstrain as you can but don't go mad, throw a walk break in every five minutes and reduce your expectations as much as you can.
I've had similar experiences myself so I know marathons *can* be finished with pretty awful prep but it is SOOOO much more pleasant if you're healthy and have got solid training in.
You've not had a lot of answers because clearly you are a sensible person who runs consistently and therefore WILL BE FINE. If you look at the HH plans you'll see they won't look that different from what you're doing already. But at the very peak of training, instead of a week where you do
3 miles, 4 miles, 3 miles, 5 miles
5 miles, 10 miles, 5 miles, 20 miles
which, er, I know may seem a lot now , but remember: if it wasn't much work doing a marathon, no-one would be impresssed .
Re undulations - there are no actual hills but there is a rise where the course exits Abingdon and heads south towards Drayton.
The last 2-3 miles (from the town centre back to Tilsley Park) are very very slightly uphill, basically because the town slopes down to the river at the centre. I would say that is not an issue, nor (I power walked it once and it took 14 seconds) is the underpass (man up, people!!). The fact that those last miles are through pretty quiet suburbs is worse!
AFAIK there are no portaloos but the foliage level is still fairly good. In 2006 (my first marathon!) a kind spectator let me use their loo, so it's always worth asking ...