Rootstock codes begin with the letters M or MM, which stand for Malling and Malling Merton, the research stations in England where the rootstocks were developed. My (highly unscientific) survey of Irish garden centres showed that M27 and M9 are the more usual types: be aware that these are the least vigorous and require the most care.
M27: extremely dwarfing (height and spread of up to six feet). Needs permanent staking and very fertile soil.
M9: very dwarfing (up to eight feet). Needs permanent staking and a fairly good soil, will not thrive with grass or weeds at the base.
M26: dwarfing (up to 12 feet): More vigorous rootstock, grows well on average soils.
MM106: semi-dwarfing (between 12 and 18 feet): vigorous rootstock, suitable for poorer soils.
Nursery notes: McNamara Nurseries, Midleton, Co Cork, holds a collection of old apples, including certain Irish varieties which can be grafted to order (ready November 1998). Inquiries telephone 021613733, fax: 021-613666.