Dublin-based drug maker Shire said it has struck an all-cash $6.5 billion deal for US biotech Dyax Corp., even as it chases a $30 billion deal for rare-disease rival Baxalta Inc. Dyax shareholders will receive an additional $4 a share if certain performance targets are met, which would increase the overall value of the deal by $646 million. Based in Burlington, Massachusetts, Dyax has specialized in the development of treatments for HAE, a rare genetic disease that can be life-threatening.