Information on the little known about St Marnock can be found here: https://en.wikipedia.org/wiki/Marnock
Detail on the monastic sites on the island of Inchmarnock are available at the Canmore website here: https://canmore.org.uk/site/40268/inchmarnock-midpark
Further information on the island itself and its history, can be found here: https://en.wikipedia.org/wiki/Inchmarnock
Here is an extract of the information on wikipedia:
Excavation to the north of St Marnock’s church revealed the remains of what is interpreted as an early monastic enclosure, together with a number of workshops and an exceptional number of pieces of inscribed slate. Provisionally dated to the 8th or 9th century AD, possibly continuing later, this is the largest assemblage of such material known from Scotland. It includes examples of abstract designs and casual graffiti but also what are clearly practice pieces for the composition of more complex designs. As ‘pattern books’ for the creation of designs in other media, the site potentially offers the opportunity of understanding better the context in which this material was produced. Many of the slates were found in association with debris from metal-working processes.
Literacy at the site was attested by a number of fragments with practice writing, as well as one example with a piece of readable text. There are clear indications that the monks who resided on the island were teaching novices the various skills expected of young monastic scholars. Among the inscriptions are examples of Gaelic, Latin and ogham script. Other inscribed slates from the site provide insights into the dress, weaponry, buildings and ship technology of the time.