Prayerful: Guítheach

Beautiful altar: Altóir álainn

Altar or war: Altóir nó cogadh

Nordic saga: Sága Ioruach or Sága Lochlannach

These are all Irish Gaelic. If it's Scottish Gaelic you're interested in, wait for Caoimhín.

Redwolf

Thanks a lot, Redwolf! If Caoimnhín drops by to contribute with the Scottish translations, I won't mind .

How different are Irish and Scottish Gaelic from each other? Please enlighten this newbie.

They're related, but different...think Spanish and Portuguese.

Redwolf

Scots Gaelic came from Irish settlers to Scotland over 1000 years ago, if I remember correctly speakers of both could still understand each other and it was even called Irish in Scotland to some extent right up until the 1800's when Scottish Gaelic formed its own written standard and started becoming more different in spelling and pronunciation.

In Scottish Gaelic:

prayerful: guìdheach or ùrnaigheach [the latter is more common when discussing religious prayer]

beautiful altar: altair rìomhach or altair bòidheach
[You could also use àlainn, but in Gaelic it's more like "fine" or "lovely"]

altar of war: altair cogaidh
[I assume you meant "of" not "or"]

Nordic saga: Not sure of the word for "saga", but it is likely to be sàga, so: sàga Lochlannach

Note that in modern Gaelic the accents all slant down to the right (they are "grave accents"), the opposite of the way they slant in Irish (which uses "acute accents").

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I'm not a native (or entirely fluent) speaker, so be sure to wait for confirmations/corrections, especially for tattoos.

Is there any reason why the accents when the other way? Was it a concious decision to be different or just developed that way?

Scottish Gaelic was written in Irish until the 16th-17th centuries, although the pronunciation, vocabulary, and some of the grammar had already diverged from Irish. Once the link to Gaelic Ireland was broken, Scottish Gaelic gradually acquired its own writing system. As it was developing (a process not yet entirely complete -- there are still competing spellings for a number of words), it used both grave and acute accents, but to represent different sounds. There was no official body in charge of Gaelic until the last few decades, though, and that made the process of change somewhat chaotic. Different people and institutions made their own choices.

I'm not really competent to explain the whole development of the spelling or sound system of Gaelic from that point on, but ultimately the acute accents were dropped. They were still there in grammars and dictionaries from the early- to mid- 20th century (including mid-century Teach Yourself Gaelic books), but they are missing from modern dictionaries. The process was haphazard, and in books from 40-50 years ago, you can find words written both ways in the same book. Even today, you will still see acute accents in things like song lyrics and proverbs from time to time (where you often also see old words which survive in Irish but are not used much in Gaelic now).

In some cases, a grave accent replaced the acute accent (Irish álainn became Gaelic àlainn), but in other cases the accent disappeared (compare Irish tá and Gaelic tha). In some cases, the disappearance of the accent makes sense, in that the vowels are not pronounced in the same way in the two languages. For example, Irish tá ("taw") has a different vowel sound from Gaelic tha ("haah"). It doesn't appear to me, though, as though there were systematic patterns followed. Just as one used to be able to find the same word with a grave or an acute accent in the same book, for some words there are now acceptable variants with or without an accent (but always a grave accent now, if there is an accent).

_________________
I'm not a native (or entirely fluent) speaker, so be sure to wait for confirmations/corrections, especially for tattoos.