Póilíní as far as I know is only used in the Irish version of 'the Northern Irish police force' which is a relatively new organisation so I can only think that this is a made up modernism. The police in Ireland are the Gardaí Síochána (the organisation), Gardaí (plural of one Garda) or Gárda (one police officer)
Irish speakers were dealing with policemen in Ireland and in the cities that they emigrated to, for a long time before the creation of An Garda Síochána in 1923, and the term póilín long predates the creation of the PSNI.
According to Garda.ie, the Dublin Metropolitan Police, formed in 1836, was called Póilíní Chathair Átha Cliath. The Scott Medal, the Garda award for bravery, is named for Coirnéal Walter Scott, Coimisinéir Onórach Póilíní Chathair Nua Eabhrac.