Right then. I'm Scottish, so here's my take:

Firstly, his accent is unmistakably Irish - which you would expect given that he was born and raised in Ireland. Kids adopt the accent of their peers, not of their parents.

Secondly, unless it's recorded somewhere there's no point in trying to guess whether he was a native Irish or English speaker. He was born in the 19th century when there were a lot more native or even exclusive Irish speakers, and a lot more exclusively Irish areas than there are now. The fact that his parents were Scottish is of little help as Scottish Gaelic was similarly more widespread at that time. And that bio says his paternal grandfather was Irish anyway. That said, his parents came from a town called Galashiels, which is in the south east of Scotland and about as far from the strongholds of spoken Gaelic as it's possible to get (and about 5 miles from where I grew up, incidentally), so whilst it's impossible to say whether or not they could speak Irish/ Gaelic, it's almost certain they could speak English.