Samples from a calf suffering tongue lesions have been sent to the world foot-and-mouth testing laboratory in Britain.
The animal's symptoms were discovered on a farm in north Co Cork last night and it has been destroyed.
Results from the samples are expected next week. It has not yet been established whether the remainder of the herd has been destroyed nor what containment measures, if any, have been put in place.
A Department of Agriculture spokesman said further details would be announced later today.
It is barely one month since Ireland was officially declared free from foot-and-mouth, and restrictions on the movement of sheep (the animal most susceptible to the disease) are still in place. A partial relaxation of measures controlling the sale of sheep in marts was announced less than two weeks ago.
The Department spokesman said authorities that control entry points to the State have been contacted to remind them to "maintain strict bio-security measures".
Over 20 confirmed cases of foot-and-mouth are still reported every week in Britain. The last incident of the disease in Ireland was reported on a farm in Co Tyrone in mid-April.