In a network-on-chip (NoC) based multicore architecture, an off-chip data access (main memory access) needs to travel through the on-chip network, spending considerable amount of time within the chip (in addition to the memory access latency). In addition, it contends with on-chip (cache) accesses as both use the same NoC resources. In this paper, focusing on multithreaded applications, we propose a compiler-based off-chip data access localization strategy, which places data elements in the memory space such that an off-chip access traverses a minimum number of links (hops) to reach the memory controller that handles this access request. We present an extensive experimental evaluation of our optimization strategy using a set of 13 multithreaded application programs under both private and shared last level caches. The results collected emphasize the importance of optimizing the off-chip data accesses.