Complex sentence: Although Irish by birth, Wogan chose to take out British citizenship, presumably because it would fully legitimise his knighthood, as hitherto he could not use the title "Sir" as a non-British citizen, since the knighthood was only honorary.
Split into simpler sentences: Although Irish by birth, Wogan chose to take out British citizenship, presumably because it would fully legitimise his knighthood. Before that time, he could not use the title "Sir" as a non-British citizen since the knighthood was only honorary.