Agree that SF is not a protest vote but they do vote and debate. Since their first major electoral victory over 100 years ago they have been clear that they represent their constituents to the best of their abilities (I won't opine on the quality of representation in this forum). That has always involved a cabinet, votes, debates, and eventually a bicameral legislature.
The key aspect is that they consider the English government to be a foreign government and so they avoid involving it in the work that they do in Ireland for their Irish constituency. Statements about the illegitimacy of their government historically come from conservative English sources. But the fact is those SF debates in their "protest government" formed the foundation of the modern Irish state. They are a protest vote in the same way that the US Constitutional Convention was.
By the same token, they consider legislating on affairs that pertain to the English, Welsh, and Scots to be none of their business. To take up seats in a foreign parliament would be to meddle in the affairs of a foreign, sovereign nation. And that would be hypocrisy!