Hi Brendan,
As far as I know, One Plan was a PTSB marketing name for a top-up loan. It's not a life insurance product.
So I think Red Head has a tracker mortgage with a top-up loan that is on a much higher rate. In those circumstances the only choices are to move everything (tracker mortgage and top-up) to another lender, which would probably be a bad idea as you'd lose the tracker mortgage, or to negotiate with PTSB for a better rate on the top-up.
Red Head - it's not possible to move the top-up mortgage to a different lender while retaining the PTSB tracker on the same house. Irish banks will only allow one lender at a time to have a residential home as security.