On an Android phone, long-press on the vowel (press and hold for a second).
On a Windows 10 PC, click Start, type "Language Settings". Click on your default language and then click on Options. You now have the option to Add a Keyboard. Some people prefer to use the US International Keyboard (press the apostrophe key, then your vowel to get the fada. That can be a hassle if you use the apostrophe a lot), some people prefer to install the Irish Qwerty keyboard (use the Alt-Gr key to get the vowel with a fada, but the @ and " keys are swapped). You can swap between the keyboards by pressing the Windows key and the space bar at the same time).