It is said that sailors would carry Hawthorn berries with them when they went out to sea. If they needed the protection or assistance from Manannan they would drop the berries into the water, sometimes after crushing them in their fingers. They would then invoke Manannan verbally and make their request. You don't have to be out to sea to use Hawthorn in ritual.
Hawthorn comes in many species, from bush to tree, but this picture or the chart below might help you identify some in your area.