The company unveiled its 13th store in the Republic last week - and numbers 14 and 15 are set to follow in the coming months.
In the past two years the company has doubled the size of its operation in the Republic. There have been eight store openings to enable it to take advantage of a buoyant economy that has almost full employment and an annual growth rate of 5%, more than twice the EU average.
The latest store opening was in the border town of Drogheda, with a 32,000 sq ft outlet offering food and clothing. Another opening is scheduled in the Dublin suburb of Tallaght in November and plans have also been finalised for a store in Athlone.
In all, M&S is planning 25 new stores over the next three years - 12 large outlets and 13 operating under the Simply Food brand - and hopes to double its 5% market share.
Business in the Republic "is running at a far higher level" than in the UK, according to local M&S executive Neil Hyslop.
"We are seeing increased confidence from our customers and the pace of growth is certainly not going to let up."