My argument is that electing a bloc of avowedly nationalist MPs does not give said MPs the right to unilaterally form their own parliament or country, because that would be breaking the law, and voting for a party in a democracy does not give either voters or the elected representatives the right to break the law. The fact they may be Irish, Londoners, Scots, Welsh, etc. is and should be irrelevant.
I reject the notion that Ireland could in any way be considered a political unit on the grounds of your own logic, because Ulster did not vote for SF and Ulster is a part of the geographic unit of Ireland.
I reject the notion that a vote for SF is automatically a vote for independence on two grounds: the first is that if I lived in Cork South and was a Unionist, I would not in any way be able to express this view at the ballot box; (and Michael Collins was not the only MP elected unopposed) the second that people have a multiple of reasons for voting for any party, and just as there are Lib Dems who oppose Proportional Representation today, there are also SNP voters who don't necessarily support Scottish Independence. I doubt that things in 1918 were any different.