I'm not sure I would class Ballsbridge as 15 minutes walk away. 15 minutes cycle maybe.
You won't get any closer that the Montrose as already outlined. The rate is pretty reasonable. The Stillorgan Park is further away, but linked by a million buses an hour at peak time, so you won't be waiting long. Alternatively a taxi would get you there quickly because of the taxi/bus lane between the two sites.
UCD is a big place though, so depending on where you are ultimately going to within the campus there might be some other suitable accommodation near the other entrances.
I don't know about the B&Bs near the Montrose - I don't think you would shave a whole lot off the €69 rate for the Montrose.
That service charge is very sneaky. I wouldn't be impresssed if I got hit with that. Any other hotel rates I have ever seen have been the full rate with the exception of local room taxes & VAT - I have never seen (and would never expect) to see a 'service charge' tagged on to my bill. And having it 15% just makes it worse.
z