I can change the IRQ priority to 51 by modifying ./kernel/kernel/irq/manage.c :
root@swi-mdm9x28-wp:~# ps -eo pid,pri,rtprio,cmd | grep irq
3 19 - [ksoftirqd/0]
20 91 51 [irq/47-cpr]
133 91 51 [irq/31-tsens_in]
140 91 51 [irq/43-7864900.]
141 91 51 [irq/42-mmc0]
148 91 51 [irq/205-modem]
1593 19 - grep irq