If only one sink is slow, the block is usually in the trap or the first stretch of waste pipe: fat, food and coffee grounds in a kitchen, hair and soap in a bathroom. If the next basin or the shower tray rises when you run it, the shared branch is the problem.
The technician checks the trap first, then the branch, and clears it by hand, with a plunger or with a small rod or jet.
Skip the chemical drain cleaners if someone is coming: they sit in the pipe and splash. Note which other outlets gurgle.
Cast-iron and clay waste pipes in older Galway houses need gentler tools than the plastic pipe in newer estates on the edge of the city.