The complain I hear most about old movies is the "slow pace" . The audience was conditions over the last decades to faster and faster action, cuts lasting a fraction of a second, jumps from scene to scene that can make one nauseous. But nothing like this happens in real life and a more pedestrian pace is not only more realistic, but much more rewarding.
So if we ignore the "slow pace" we have here a good solid story slowly reaching its solution. And really everything is going in favour of the movie: good acting, good photography, authentic locations. And like any movie from some 60 years ago, it is an interesting document, a time capsule with the real objects (not fake props), the real customs and details. We usually do not see this in the anachronistic period movies made decades later.
From the very first image of the classic British binoculars, the WW2 Bar & Stroud Cf41, to the scenes of the Dublin airport with the Vickers Viscount airplanes - it is all as authentic as it can be for the early 1960s.