Refer to these links to get a feel for both languages

I speak English & Irish and as my joint 1st languages. I am reasonable in spoken and written French and Spanish, less so in Italian and German and studied Latin in school.

Irish grammar is very difficult as it has five declensions of nouns with the following cases for singular and plural: Nominative, Vocative, Accusative, Genative, Dative.
There are changes to the beginning and the end of nouns to reflect these and also changes internally.

Verbs are no more difficult that Spanish, although the structure does not follow the Latin grammar structure so familiar to languages descended from Latin

Irish is a sister language of Latin and Greek so there are many similarities between the roots of many words but sometimes these are not obvious at first sight
eg
Man – Fear – Vir(Latin)
Sight – Fis – Vision(Latin)
Horse -Capal – caballus(Latin)
Steed – Each – equus(Latin) – Hypo(Greek)
Woman – ban – Gyn(Greek)
Bright – geal – gala(Greek)
Bull- Tarbh – Tauros -(Latin & Greek)
Cow – Bo – Bovus(Latin)

English Irish Latin
One Aon Unus
Two Do Duo
Three Tri Tres
Four Ceathar Quattuor
Five Cuig Quinque
Six Se Sex
Seven Seacht Septem
Eight Ocht Octo
Nine Ne Novem
Ten Deich Decem
Eleven Aondeagh Undecim
Twelve Dodeag Dodecim

There are many other examples.

Irish is a Q Celtic language while Welsh is P Celtic,
ie P sounds are normally changed to Q or K sounds in Irish.

See – http://en.wikipedia.org/wiki/Celtic_languages

I know little of Welsh but from that what I have seen of it, I find it completely incomprehensible with very little similiarity although there must be a lot of unseen similiarity at the "root" level.

For those who study comparative linguistics a knowledge of a Celtic language is useful.

They both have the distinction of being the first written European languages north of the Alps, also many Latin writers were of Celtic origin Virgil being the most famous.

Irish fear geal = bright or blond man

The one you will find easiest is the one you have most motivation to learn. If you have access to S4C (Welsh-language TV) or some Welsh-speaking friends, Welsh will be easier. If you spend holidays in the Gaeltacht, or have access to RTE – Irish might have the edge.

The differences between North and South Welsh are much exaggerated – largely it is just a matter of a few dozen common vocabulary words. You should learn whichever form your friends speak (or the one you can get the best learning materials in).

Edit…Oh, I recommend North Wales Welsh…at least we know how to mutate properly…..I can help you if you like.

In reality, the differences aren’t that great, there are a few words that are different in the North and the South eg:
funnel….twmfat (N)…twndish (S)
out…..allan (N)…màs (S)
up…i fyny (N)…lan (S)
dirty…budr (N)….brwnt (S)
milk…llefrith (N)…llaeth (S)
to swim…nofio (N)…oefed (S)
ice…..rhew (N)…iâ (S)
egg…ŵy (N)…wŷ..(S)
busy…prysur (N)…bishi (S)
untidy…blêr (N)…anniben (S)
nappy…clwt (N)…cewyn (S)
cattle…gwartheg (N)…da (S).

and the South can’t distinguish between ‘u’ and ‘ i’ (eg: they pronounce ‘mul’ [donkey] and ‘mil’ [thousand] the same but we don’t) so the accent is different too.
When they say ‘twyll dyn’ (man’s treachery) it sounds very much like ‘twll dîn’ (ar5ehole)…lol.
It’s similar to the difference between the Portuguese spoken in Portugal and that spoken in Brazil or British English and American English.