As a rule, no. New development is required to keep surface water out of the foul sewer, and Uisce Éireann's connection standards and building regulations both work on the basis of separate systems. Some older urban areas have legacy combined sewers, but connecting new rainwater run-off to a foul sewer is not normally permitted.
Foul sewers and treatment plants are sized for wastewater flows, not storm flows. When rainwater is added, heavy rainfall can overwhelm the network, causing sewer surcharge, flooding of properties and discharges of diluted sewage from overflows. For that reason surface water connections to foul sewers are refused for new development.
In parts of older towns and cities, historic combined sewers still carry both flows. Even where a combined sewer exists, the current approach is to separate new flows and to manage rainwater on site wherever ground conditions allow, using soakaways, attenuation or other SuDS measures.