Actually a tern, not a skua.
We watched it hover above the Lagan before diving headlong into the green depths, but it soon appeared fish-less on the wall. I had an epiphany on the bus when I realised that I had misidentified it as a skua. Silly me.
Note the forked tail, as opposed to the flat tail of a gull. It is often written that terns are known as "the swallows of the sea", but that seems like one of those "facts" contained only in books. I don't know anyone who would use such a trite description.