Many spiders are very similar, and there is a lot of worry of some of them due to the colonisation of Ireland by False Widows in such numbers these days. The spider that is mostly mistaken for a False Widow is the very common Missing-sector Orb-weaver, Zygiella x-notata, which is found around the windows of houses in huge numbers. Zygiella doesn’t get to be as big as False Widows can, but since many False Widows encountered are not fully grown, then confusion is inevitable. What Zygiella does is spin a classic spider-web across the front of windows so that insects flying to the windows at night will crash into them and become entangled.. Most orb-weaving spiders spin their webs across fly-ways in the same way that poachers cast nets across rivers. But remember, the False Widows don’t do this because they target different prey. So the dozens of spiders camped around the outside of your window-frames will almost certainly be Zygiella and not False Widow. But they have a very similar body-shape and markings on the back can be superficially similar. Zygiella is usually much brighter and tends to have a silvery appearance.
On the other hand, the male Steatoda grossa False Widow, which has been recorded in Ireland since the late 19th century, can be mistaken for a House Spider, as it has very long legs and tends to scuttle along the ground and will enter houses simply by walking into them, and in this manner is extremely different to the more robust-looking Steatoda nobilis male. Unlike the females, they are not known to bite, but these spiders can get quite large (about as big as a mediu-sized House Spider) and will frighten most people. The markings on the abdomen have a checker-board appearance like the classic markings of the female.