I don't think the action is racist, just shortsighted. I am assuming the banning of the turban is on the grounds that alternative head wear is not permitted, to include baseball caps and woolly berry's, not that there is a racist belief that people who wear turbans as opposed to western headgear are any less able.
Shortsighted because yes, the Garda can be a stickler on uniform, but this may result in a less than representative force. The Met, presumably overcame the traditional dye in the wool beliefs by realising that having a little modern flexibility, allows for a wider population to be recruiting officers from. This helps to be able to have a choice of candidates to interview and some can be rejected rather than taking on anyone to put bums on seats. Also I understand having a diverse police population reflecting the wider population makes for improved policing.