It is not daft, and is exactly what I am doing in July, although flying to Dublin from Manchester with FR. Cost wise you will need this as two separate tickets but you could have your DUB/LHR or LGW/MCO nested within your NCL/LHR/DUB return but easy enough to book as two tickets. Is there a reason why you want to fly BA via LHR to Dublin?
Can I also suggest the following:
Flight 1 DUB to LHR or LGW
Flight 2 LGW to MCO
Flight 3 MCO to LGW
Flight 4 LHR to NCL
Flight 5 LHR to DUB
You would simply not show for the final flight. This is doable and adds only a small cost to cost without flight 4. Will mean you need to transfer from LGW to LHR but saves flying to and from Dublin again.