The westernmost point where UTC with DST is applied is El Hierro, Canary Islands, Spain (18°00' W). Time used there is 2 hours and 12 minutes ahead of physical time in the summer, making for the greatest discrepancy in the UTC time zone.
Note that Ireland uses the term "standard time" in reference to the summer months and "winter time" during what the rest of the time zone calls "standard time". This is the reverse of the practice of most countries in the EU, but provides the same end results.
The westernmost point where UTC with no DST is applied is Bjargtangar, at the northwest peninsula of Iceland (24°32' W). Time used there is 1 hour and 38 minutes ahead of physical time. This is the greatest deviation from physical time for UTC with no DST.