With regard to why Gibson-Park is starting at 9 for Ireland, it's been a controversial enough call in Ireland. Or at least it was until his performances against Japan and New Zealand in November quietened many of his critics. He's got the skill set that Farrell wants in his scrum-half, can get the ball away quickly, provides enough of a running threat, excellent support runner. His performance on Saturday was a bit hesitant though and that will be punished by better sides.
On the other scrum-half options in Ireland, Conor Murray used to be head and shoulders above all the others but he's regressed quite a bit since he picked up a neck injury in 2018. His understudy at Munster, Craig Casey, certainly has the zip to play the type of game that Farrell wants and I hope Munster bring in a head coach who wants his side to play some rugby as this will definitely benefit their international aspirations. Van Graan's bok-lite rugby at Munster has killed some players' international ambitions down there.
Luke McGrath starts as many matches as JGP for Leinster when they're both fit but his passing accuracy simply isn't good enough for international rugby. A shame because he's a better player than JGP in most other aspects.
Couple of decent options at Connacht in Kieran Marmion and Caolin Blade but their faces don't seem to fit, same with John Cooney of Ulster who was the form scrum-half in the country in 2019 and 2020.
Best prospect is Nathan Doak up at Ulster. Only turned 20 in December, already getting plenty of starts for Ulster due to Cooney's injuries. Looks the heir apparent and certainly is chippy enough. Actually might need to tone it down a bit as he's pissed off some refs in the last couple of matches.
To sum up, Gibson-Park is one of a number of decent but not top-class scrum halfs in Ireland. He's got the jersey at the moment but it wouldn't be a massive surprise if he disappeared off the radar completely if his form dropped a bit.