Kit Harington (Michael) and Jack Reynor (Declan) have a discussion about medieval studies and Declan asks Michael if he means Game of Thrones. Harington played Jon Snow in Game of Thrones (2011-2019).
Lucy Boynton and Jack Reynor also starred in Sing Street (2016).
Declan's "vintage" car is a Peugeot 309, a model produced between 1985 and 1994. So yes, as of 2020, it is quite vintage.