The beautiful, ivy-covered ruin of Menlo Castle with its sad story are on private property. Still, many climb the gate to see it up close. If that's not your thing, you can still enjoy the sight from the opposite river bank.
Galway Tourism (Menlo Castle, 2022-10-06):
galwaytourism.ie/menlo-castle “Please note: Menlo Castle is currently closed due to ongoing conservation building work (updated May 2022). ...”