TaxHelpPlease!

Hi there! I worked in the UK over the summer and made well-below the personal allowance, but unfortunately my income was taxed at BR. I wasn't able to change this in my tax app as I was unable to get a national insurance number, despite going to a job centre to get one. I have since left the job and have returned to Ireland, and I will not be working in the UK for the remainder of the tax year. My former employer sent me a P45 as I left, and it says that my tax code upon leaving is still the BR. Is there any way that I can change my tax rate and claim my tax back, as I only made £4,000 and will not be returning to the UK this year?