In the second example, “so…that” indicates a cause-and-effect relationship. The construction of “a £ 5 million manufacturing plant in Ireland ” is the result of their confidence. The infinitive “to make” indicates the purpose of constructing the plant. The information between commas refers to “a similar solid version” and the clause introduced by “which” gives extra information about “Pop Rock”. Thus, structure-wise, the main idea is “They are so confident of its success.”
Example 2
They are so confident of its success that they have recently constructed a £ 5 million manufacturing plant in Ireland to make the dust and a similar solid version of the experience, called “Pop Rock”, which will initially be test-marketed on the continent.