Because, tony, 209, as with all 201s, is part of a shared pool. As such, IE or NIR can use each other's 201s as the need arises. As IE have a bigger need for locos, naturally 209 often ends up on the Irish network. As well as that, prior to the addition of MK3 vans, 209 could not work DD stock as her HEP equipment is not operational, hence was not useable for the Belfast link