Ireland is a large (84,421 km2) island, off an island (Great Britain), off a continent (Europe), with a depauperate yet unique mammal community. It is vulnerable to non-native species introductions and all mammalian guilds have been invaded expect the bats. The origin, spatial extent, expansion rates and impacts of two invasions will be compared; one involving a relative large (4 kg) mammal, the European brown hare (Lepus europaeus), and another involving two small (< 20 g) mammals; the bank vole (Myodes glareolus) and greater white-toothed shrew (Crocidura russula). European hares were intentionally released during the 1800s for field sport and now occupy a relatively small range (535 km2) which expanded 3-fold between 2005 and 2012-13 (0.73 km year-1). They are replacing an endemic species of conservation concern, the Irish hare (Lepus timidus hibernicus), through competition and hybridisation. The bank vole and greater white-toothed shrew were introduced during the early and late 20th century respectively; both unintentionally. They occupy a substantial range (32,700 km2) and are expanding quickly (2-6 km year-1) associated with the decline of the native woodmouse (Apodemus sylvaticus) and extirpation of the pygmy shrew (Sorex minutus). Due to their body size and scales of invasion they pose different management challenges. The native hare could be protected by culling the invader with eradication possible albeit difficult. The invasive small mammals are naturalised and impossible to remove but native small mammals may be protected by forward planning of landscape composition and land use change. Management strategies will be contrasted.