A "small number" of Irish firms are likely to see their credit ratings downgraded if neighbouring Britain, Ireland's largest trade partner, exit the European Union (EU) in March without a deal, S&P Global said on Thursday.

The agency noted it still expected the UK to leave the EU with a deal but said the risks that it doesn't were growing. 

"A no-deal Brexit would have negative credit implications and place increased pressure on Irish issuers," S&P said. "However, we would only envisage rating actions for a small number of issuers, where rating performance is already somewhat challenged."

According to the agency, the agriculture sector would be hardest hit by a no-deal Brexit while banks were unlikely to see many "near-term" rating moves, and a downgrade of the Irish sovereign was not its "base-case" in such a scenario.

S&P currently rates more than 50 companies in Ireland as well as the government.