If you decide not to use a collection agent, this is how it will work: instead of collecting 100% of your rental income and then paying out 20% tax on these earnings at the end of the year, you can arrange that your tenant pays you 80% of their rent, withholding the 20% to pay directly to the Revenue Ireland on your behalf.
This often works best for long-term and trusted tenants, or for landlords and tenants with an established relationship (such as family members or friends of the family).