You have three options as I see it, from your question:
You want points within the triangles of your mesh as suggested by underdark:
Go to either Vector->Research Tools->Select by location and complete the dialog box options appropriately, OR, go to PlugIns->Spatial Query and select appropriate options there.
You want points that lie within the extent of all the triangles.
Go to Vector->Polygon from layer extent and make a polygon of the full extent. Then follow the steps in option 1 above using your new polygon.
You want points that lie within the sum of all your triangles (like option 2 but excluding those areas not covered by triangles.
Do option 1 - it's the same thing but just phrased differently
From the way you phrase the question, I think you want option 2 but without clarification I'm not sure.
Hope this helps