Of course, "tion" (shun) as a combination of letters represents a consistent sound in a way that "ough" does not. In Irish also you often have to look at two or three letters together to know what sound they signify. But the rules are generally straight-foward enough; for instance s before i or e has an sh sound (sí is like the English word she and seo like the word show), while s before an a,o or u is pronounced like the s in 'sound'. There are also letter combinations which show the derivation of the word rather than the sound.* For instance, "cruinniú na mbad" (gathering of the boats - regatta) - the b isn't heard but the spelling shows that 'na bad',of the boats, comes from the word 'bad', boat. Once again, this is not as difficult as it seems for the letter combinations are limited and consistant and governed by rules or grammar.

*Oh, the fh of Lasairfhiona is silent, but written to show that fhiona derives from fiona.