I believe it is very much like the difference between Spanish and Italian where native speakers can understand each other up to a point, especially for basic everyday conversations.

I have also heard of proficient non-native speakers going to the Hebrides and being able to converse with their Irish while the locals used Scottish Gaelic.

On the other hand, it would only be a smallish step over to Scottish Gaelic after you have learned Irish Gaelic, but it would be really confusing to try to learn them both at the same time.

(That's my two cent's worth, but I'm not a Scottish Gaelic speaker - maybe one day, heehee!)

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Múinteoir Gaeilge - Irish Teacher
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect, but I can also speak Ulster and Munster Irish with native-level pronunciation.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), Gaeilic Uladh (GU), Gaelainn na Mumhan (GM), agus Gaeilge an Chaighdeáin Oifigiúil (CO).

Breandán's right about the differences/closeness.

The majority of the vocabulary is essentially identical, with some spelling differences which are fairly easy to get used to, since they usually follow regular patterns. Some of the words still used in modern Irish sound old-fashioned to Scottish Gaelic speakers, whereas some Scottish Gaelic spellings of words look old-fashioned to modern Irish speakers. Irish underwent a large spelling modernization in the mid-20th century, and Scottish Gaelic spelling is still somewhat in flux, with many variant spellings.

Most of the grammar is also the same, although Scottish Gaelic arguably has a more simplified verb system (and definitely a simplified system of pluralizing nouns, which can be complicated in Irish) and each language has some forms different from the other when it comes to prepositions and pronouns. Sometimes, though, an apparent difference seen in the written language is not so different as actually pronounced. There are some strong differences in pronunciation which can make it hard to understand one another, but native speakers can probably work out most basic things. Scottish Gaelic has maintained more of the old guttural sounds, while some of them have gone silent in Irish (and have even been deleted in the modern spelling in some cases), and the quality of vowel sounds can be very different. Scottish Gaelic also has several pronunciation twists inherited (probably) from Norse which have altered many words.
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I'm not a native (or entirely fluent) speaker, so be sure to wait for confirmations/corrections, especially for tattoos.

While we're waiting on a third confirmation, I have a question. I heard a story a long time ago, it had something to do with the Celts sacking Rome, and during some kind of verbal confrontation between delegates, the Celts told the Romans "To the brave come all things" which I believe in Irish is "Is leis an cróga gach ille rud". Does that sound right? I also had a cousin who used to tell me "leabhar bearla loim, agus brisfidh mé dó phus", which I believe meant "if you speak English to me, I'll break your face", but again, I'm not 100% sure. Its been a long time since I've seen or heard it (he had a t-shirt made for me with that on it). Also, for grins and giggles, here is the tattoo I have on my right shoulder, I've actually had more color added since this picture, I had the white brightened up and the light green on the inside of the shield was darkened a bit. I'm going to get quite a bit more work done in the next several months, I'll post as I do.

Cool tatt ! Pictures are always welcome.

Is leis na cróga gach uile rud might work for "To the brave come all things" or literally "the brave own all things" ( "the brave shall inherit the earth" )

Labhair béarla liom agus brisfidh mé dó phus "if you speak English to me, I'll break your face" literally "Speak English to me and I'll break your snout".

Await correction or confirmation ...

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Múinteoir Gaeilge - Irish Teacher
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect, but I can also speak Ulster and Munster Irish with native-level pronunciation.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), Gaeilic Uladh (GU), Gaelainn na Mumhan (GM), agus Gaeilge an Chaighdeáin Oifigiúil (CO).

I know we prefer Irish around here, but I'd still give Béarla a capital letter!
Also, do phus (no fada).

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Thanks. I didn't see the síneadh fada there.

Not sure béarla deserves a capital in Irish, though.

Since we've got corrections on corrections, here are the fully corrected versions (probably):

Is leis na cróga gach uile rud

Labhair Béarla liom agus brisfidh mé do phus

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Múinteoir Gaeilge - Irish Teacher
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect, but I can also speak Ulster and Munster Irish with native-level pronunciation.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), Gaeilic Uladh (GU), Gaelainn na Mumhan (GM), agus Gaeilge an Chaighdeáin Oifigiúil (CO).

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I thought it was Germanic tribes that sacked Rome, not Celts...

Redwolf

No the Celts sacked Rome also, headed by Brenus. Brenus was also the name of a Celtic king who sacked the sacred Greek city of Delphi. They think they were the same Brenus though. Some also think breandán is derived from the name brenus.

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Please wait for corrections/ more input from other forum members before acting on advice

Ya a good bit of difference in pronounciation, however proper spoken irish is very gutteral also- I suppose Ulster Irish is an exception. Unforetunetly people aren't thought proper pronounciation in school and people pronounce words with an English influence so bukill for buachaill, nowadays, which really annoys me!

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Please wait for corrections/ more input from other forum members before acting on advice