Many of the oversubscribed Catholic secondaries say you have a duty to give your child a Catholic education, and having attended a Catholic primary, along with having done first holy communion, and mass 3 times per month plus holy days of obligation is a requirement.
For a faithful Catholic, this is not hard to do! The priests reference that will support your application will outline any amount of voluntary work or ministry within the Church, be it cleaning, Reading, flower arrangements, gardening, etc. This will count when the secondary has more applications that fulfill the criteria than places.
But for a lack-luster fair-weather "Catholic" this becomes harder.
Many of the not very Catholic children from my dc primary did not get a Catholic secondary place. (Us included, as our son was not baptized within 6 months - this is a top criteria)
So, for a primary place, your child will need a Catholic baptism (primaries are often not so strict regards to how soon after birth your child was baptised) and a reference from a Catholic priest.
Unless you go for a community place, which will be around 10% of the intake for many Catholic schools, and then the geographic catchment will count in your favour - ie you live very near.