A level that falls slowly points to a blockage in the pan or just past it: wipes, too much paper, a small toy. If the shower or bath gurgles when the toilet is flushed, the block is in the shared pipe below them.
The technician tries the pan first with a plunger or auger, then moves to the branch pipe if the toilet clears but the trouble returns.
Close the small isolation valve on the cistern feed if there is one, so it cannot refill, and say whether there is a second toilet in the house.
Older Galway terraces often share drain runs with the house next door, so a toilet that keeps blocking may have its cause on the other side of the wall.