You assign the ToolTIp the same way you would any other property
<asp:repeater runat="server" id="myRepeater" xmlns:asp="#unknown">
<itemtemplate>
<asp:button runat="server" text="<%"> ToolTip=<% Eval("btnTooltip") %> />
</asp:button></itemtemplate>
</asp:repeater>
myRepeater.DataSource = myDataSource;
myRepeater.DataBind();