I have been advised by the Irish Naturalisation and Immigration Service (INIS) that it is not the practice to extend a permission to remain to persons admitted on holiday visas for a period of 90 days or less, save in very exceptional and unforeseen circumstances. The reason for this approach is that an extension of visitor's permission should not generally arise in circumstances where the person has accurately and fully disclosed their purpose for intending to come to the State at the visa application stage and/or to an immigration officer at port of entry.
Where exceptional and unforeseen circumstances arise, the person may contact the General Immigration Section of INIS to seek to extend their stay in the State.