'I have yet to see people who worked so hard for so little gain. It meant incessant toil with the spade" was how the painter Paul Henry described his first impressions of small farmers on Achill Island in 1910. His oil-on-canvas The Potato Diggers , depicting Michael and Margaret Toolis working in the harsh landscape, will go under the hammer at Adam's in Dublin on Wednesday.

Auctioneer James O’Halloran described it as “a masterpiece and the best Henry that has ever come to market”. The estimate is €250,000-€350,000 and the vendor is an Irish family who acquired it directly from the artist in the 1930s for around £300. The painting was last seen in public 20 years ago when it was loaned to an exhibition in Dublin.

Two other paintings have estimates in excess of €100,000: Thatched Cottages with Lake and Mountains Beyond by Paul Henry (€120,000-€160,000); and A Grey Morning in a Breton Farmyard by Walter Osborne (€100,000-€150,000), which depicts Keramperchec, a hamlet near Pont-Aven in Brittany.

Other highlights include a further three smaller paintings by Paul Henry; rural scenes by Frank McKelvey; and a selection of watercolours by Mildred Anne Butler, mainly featuring creatures great and small in the grounds of her Kilmurry House and, perhaps most interestingly, her sketchbook containing seven small watercolours (€2,000-€4,000).

Other notable lots include a wonderfully sunny summer picture, Invitation To Go Haymaking by Richard Thomas Moynan (€20,000-€30,000); A View Of Derry Through A Bank Of Poppies (watercolour) by Andrew Nicholl (€7,000-€10,000); and Lake Island by Patrick Hennessy (€4,000-€6,000).

A sculpture section includes a bronze Bust of Donagh O'Malley by Irene Broe, with an estimate of €800-€1,200.

Viewing begins at Adam's, 26 St Stephen's Green, tomorrow at 2pm.

More Henrys at Whyte's
Whyte's art auction on Monday also features Paul Henry whose pencil drawing A Connemara Woman is estimated at €3,000-€5,000 and an early oil, Connemara Landscape , is €8,000-€10,000. Auctioneer Ian Whyte said there was increasing interest from collectors for traditional, representational art and that "worthy contenders in this genre" include Frank McKelvey's Gypsy Caravan (€10,000-€12,000) and Fair Day Scene (€12,000-€14,000).

Other notable lots include: Tony O'Malley's Island of the Arawaks (€12,000-€15,000), a colourful painting inspired by his winter visits to his wife's family in the Bahamas; Currachs Off Roundstone by Gerard Dillon (€25,000-€35,000); and Alan, Son of Dr Martin Dempsey , a portrait of the son of a wealthy Dublin doctor on a fishing trip, dated 1917, by Leo Whelan (€5,000-€7,000).

Viewing from today at the RDS, Anglesea Road . Videos describing some of the key lots are on whytes.ie. Auction begins at 6pm on Monday.