Confidence: I'm as sure as I can be.
Notes: Peadar na Breac gave me the lower jawbone he found at a local hill lough on 1st March. He posted it on iSpot as linked in comments below. I visited the same lake the following day and quite luckily found the upper right maxillary from the same fish at the site(last 2 pics). Just got round to photographing the two together today. The left side of the upper jaw has been badly chewed. Only resident brown trout occupy this lake, there are no migratory salmonidae nor pike in it. From a large male(elongated and hooked profile).