It's kind of the same here in Dublin with a Leap Card regarding different ways you tap in or out...
On Dublin Bus and GoAhead Ireland you only tap on a device located on the right of the entrance door.
On BusÉireann you tap on the ticket machine when you've told the driver where you are going and it debits the correct Leap fare.
On the Luas you tap in on a reader on the platform and tap out again when you leave the platform.
On DART, Commuter and some Intercity Irish Rail services you tap on either the ticket barrier or a reader on the platform before you board and do the same when you leave, unless you need to purchase a ticket on board where you tap the guards machine.
The Luas, Irish Rail, Dublin Bus and BusÉireann don't take contactless card payments for tap on/tap off, but buses do now take contactless (only Irish issued cards though) for payment of tickets. GoAhead Ireland take any card for ticket payment but not tap on/tap off.
The take up over here of contactless payments is lower than it is for the UK, and a lot of people still pay in cash for most things.
Like London, Leap fares are cheaper than cash, and the TFI 90-Minute Leap fare means you can tap on/off as many times as you like for 90 minutes from the first tap and only be charged the maximum fare of €2.