Roots find the joints in older clay pipe and grow into a mat that catches everything else. The usual approach is to open the inspection chambers, find where full meets empty and clear between them with rods or a jet, with a camera afterwards if roots are suspected.
The technician lifts the chambers to see which are full, then works out where on the run the block sits.
Note trees, hedges or any digging near the line, and whether this has happened before at the same time of year.
Past the boundary the pipe may be public sewer, which makes it a matter for Uisce Éireann rather than for you.