I've done some test and this approach works perfectly:
<p:dataTable var="entry" value="#{....}" styleClass="myTable" rowIndex="rowIndex">
<p:column headerText="Header 1">
<h:outputText value="#{entry.value1}" />
</p:column>
<p:column headerText="Header 2">
<h:outputText value="#{entry.value2}" />
<pe:tooltip value="This is row number #{rowIndex}" forSelector=".myTable tr[role=row][data-ri=#{rowIndex}]"
shared="true" atPosition="top center" myPosition="bottom center" showDelay="500" />
</p:column>
</p:dataTable>
Note that in order to the data-ri attribute to be placed on datatable rows, you need the to add the attribute rowIndex (rowIndex="rowIndex").
It also worked with
<p:tooltip for="@(.myTable tr[role=row][data-ri=#{rowIndex}])" value="This is row number #{rowIndex}" />