You wonder why nobody had thought of it before. Made for TG4 with assistance from the Irish Film Board, Men at Lunch (largely in English) investigates that famous photograph of 1930s construction workers eating their sandwiches on a girder many storeys above New York City.

Seán Ó Cualáin does an excellent job of grouping together a potted study of the immigrant experience with an examination of the photograph’s history and an analysis of the various Irish connections. There are surprises and moments of tension along the way. Fionnula Flanagan narrates with all the fruity resonance we expect from that fine actor. The revelations about the practices of early daredevil photographers will induce vertigo in all but the bravest psyches.

All that noted, the film does not really scream out for commercial presentation. A fine piece of television that won friends at festival screenings, Men at Lunch really belongs in a prime spot after the evening news.

The tension comes from a suspicion – convincingly dispelled – that the image might be a fake. The film-makers travel to a deeply buried archive, locate the original plate and invite us to breathe a collective sigh of relief. Then Ó Cualáin sets to his main task.

Historians of New York have had a great deal of difficulty identifying the lofty workers. But our current film-makers feel sure that two of the men were immigrants from the town of Shanaglish in Galway. By connecting that remote village with the building of a mighty tower in New York City, Ó Cualáin creates a rather gorgeous arc that sweeps across the myth of Diaspora. The eventual conclusions are as neat as they are moving.

At times, the film does somewhat overreach itself. The core image is striking, but it’s not quite as “iconic” as claimed. The allusions to 9/11 are unnecessary. But Men at Lunch remains an honourable, worthwhile effort.