The castle, near River Irvine and overlooking Irvine's oldest street, Seagate, is an important part of the town's heritage.
For this phase of the investigation, the council is working closely with Historic Environment Scotland (HES) and has instructed works to carry out localised excavations and analysis of the castle grounds under the supervision of a specialist archaeologist team.
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This is necessary for the Council to comply with all Consent Conditions imposed by HES and will inform final decisions relating to any subsequent structural stabilisation works.
The aim of the project is to preserve the longer-term position of this very historic and important heritage asset.
Works started on Monday, June 5 and are expected to take up to four weeks, depending on the weather conditions and any findings.
Councillor Tony Gurney, Cabinet Member for Green Environment and Economy, said: "As custodians of this important building, it is the Council's job to help preserve it and protect it.
"Working in partnership with HSE, we will ensure that the castle is maintained for future generations."
The third and final version of the castle, which is owned by the Council, was built in 1565 by Hugh Montgomerie, 3rd Earl of Eglinton as a home for him and his wife Agnes Drummond.
The family arms can be seen on the ceiling bosses inside the front doorway.
Seagate was abandoned as a residence in the 1740s and in 1945 was gifted to Irvine Burgh by former owner Mrs Jessie Walker.
