I'm reading a book at the moment (The Vikings: Voyages of Plunder and Discovery) and it mentions that the story of Ragnar being captured in battle and the whole snake pit thing around the 850's. It then goes on to say that king Aella may not have been in power until 866 and Ragnar is likely to have been killed by a Norse king in Ireland. What do we know about this?
Ragnar may or may not have been a historical figure. The sources available to us place him right on the edge of a sort of legendary, imaginary past, alongside bunches of Scandinavian kings like Halfdan the Black, the Scylfing clan and others. It's been suggested that Ragnar might be a composite character - a mash-up of various real historical people.
As for whether or not he was thrown into a pit of snakes, if he ever even existed: almost certainly not. There aren't bunches of the critters in northwestern Europe, and lots of these legendary kings meet almost comically grisly ends, like the Yngling dynasty of Swedish kings who supposedly drowned in vats of mead, were sacrificed to trolls and met other such outlandish fates. It's a literary trope.