The only way to do this is to sell your shares, pay your Capital Gains Tax if applicable, invest the proceeds into a PRSA and claim your tax relief. The two transactions are unrelated.
Even if you do have a self-administered pension arrangement (which is rare in PRSAs) you can't sell any asset to your own self-administered fund so again you'd have to sell the shares, put the money into your pension and then instruct the self-administered arrangement to buy more shares, assuming you're talking about publically quoted shares.