In response to some of the posts above - yes, the Tyrells may be arrogant and overconfident, but I think the Ironborn are too few and their combat style is more suited to hit & run raids than head-to-head combat. The Ironborn have two chances of winning - luring the Tyrell army into a huge trap/ambush or having dragons allied with them. I don't think Euron would be able to count on his brother getting back with the dragons on time, So, that leave some sort of trap or ambush.