Think I have finally sorted out the mystery of this painting, after another query. The painting, called "The First Irish Pilot" is of Lavery's daughter Eileen who was married to the Master of Sempill, both of them pilots (see "Sir John Lavery" by Kenneth McConkey, page 162). Another painting "'The First Flight to Dublin" also dates from this period (1926); the Irish Free State had bought a few planes from the British and these were flown to Dublin by the Sempills .
The Lavery portrait of Lady Mary shows her in her ambulance driver's uniform standing beside a car. It was painted by Lavery when he was on war artist duties in France and remains in the house of a distant relative in Co Limerick.
From the Hugh Lane Art Gallery Website.
An Irish Pilot
This fine portrait is in good condition, but its valuable, ornate frame is suffering from extensive flaking of the gilding. This requires urgent attention from a gilding conservator before it can be displayed. Intriguingly, the portrait appears to have been painted over a different composition and we hope to investigate this fascinating possibility by x-raying the painting.
Conservation costs €3000