Haven't seen much of Foley, but when I have seen him I haven't been impressed.
That said, I trust MIW's opinion and hope Foley is as good as is being described.
While I am a fan of McShane, and want him to do well, I don't really see him as a fullback. For me he should be on the bench as centre-half cover at this stage.
With the current injury situation, I'd be prepared to give Foley his chance over McShane, but I believe that Trap will go with the latter.
All that said, I've been wrong before (Whelan), and hope that whatever decision Trap makes will be the right one.