Irwin was an archaeologist and a member, and sometime secretary, of the Cornwall Archaeological Society (CAS) from its inception in 1962. She made a Cornish bard in 1979, taking the name Maghteth Hendhyscans, meaning 'Servant of Archaeology'.
When she joined the Society in 1962, she was living in Bodmin and acted as the archaeological correspondent for that area, becoming the secretary of the Society's Area Correspondents in 1968. Her knowledge of Bodmin Moor was extensive; her observations at Helman Tor led to Roger Mercer's excavation and discovery of the second Neolithic hillfort in Cornwall.
She was the Honorary Secretary of the Cornwall Archaeological Society between 1974 and 1987 and was instrumental in establishing the Cornwall Committee for Rescue Archaeology, acting as its secretary from 1975-1987. She was elected as a Vice-President of the Society in 1988. An obituary was published in issue 33 of the journal Cornish Archaeology.
Irwin continued her own archaeological work. In 1975, she published an investigation of in on an obscure earthwork near Restormel in the CAS journal and in 1976, published her watching brief report (a report on a method of archaeology used to preserving archaeological remains in the face of development) on the building of the Bodmin bypass. Her particular interest, based on her degree in geology, was in stone tools and artefacts.
She retired to Wells, Somerset, but maintained close connections with the CAS.