. . . It's also pretty ironic that he went into space exactly 100 years to the day after the start of the American Civil War.
A mildly interesting coincidence and irony are two different things.
I think you'd be right if Gagarin's flight were 100 years after the end
of the Civil War, but I think the first human spaceflight coming on the 100th anniversary of the start of an important war qualifies as irony.