Techically, when a Filipino chooses to acquire a new nationality, i.e. Irish or British in my case, he/she is renouncing her Filipino nationality. In the eyes of the Philippine government, he/she is now a foreign national. If he/she wants to reacquire the Philippine nationality, he/she must apply for re-acquisition of it (it's in the Philippine embassy website if you want to check the process and documents needed). This will make him/her a dual citizen, i.e. of Ireland and Philippines. But not all countries allow such things. I believe Japan doesn't permit its citizens to have dual nationality (a friend has a Japanese colleague who told her that).