A tel link, read by every phone camera. The phone shows the number and asks to confirm the call, so nobody dials by accident. Use the international format (+353 for Ireland) if people from abroad might scan it.
This is the smallest code you can make, which means it can be printed small or decorated heavily. It is also the one case where a frame text really earns its place: "Call us" beats a bare code on a van door.
Questions
Does the call start immediately?
No. Both iPhone and Android show the number first and the person taps to call.