Complex sentence: Griffith may have initially come to An Stad in pursuit of furthering his favoured early cause the Gaelic League, but he had already formed a pro-Independence mindset, and not long after his documented time at An Stad, he founded Sinn Féin.
Split into simpler sentences: Griffith may have initially come to An Stad in pursuit of furthering his favoured early cause, the Gaelic League, but he had already formed a pro-Independence mindset. Not long after his documented time at An Stad, he founded Sinn Féin.