All of the modern Celtic languages form the equivalent of a present participle by prefixing a (more or less gramaticalised) preposition to the verbal noun (gerund??)
Welsh uses 'yn' [(ə)n] which otherwise means "in" but without the nasal mutation of the preposition 'yn/ym/yng'. So 'yn mynd' "going" might be thought of as meaning "in (the act of) going" etc.
The Gaelics all basically use 'ag' (OI 'oc') which as a preposition means "with/at/by". In Scotland as a normal preposition this has become 'aig' [εk] probably by analogy with the preposition-pronoun combination 'aige' "with him/it". Before verbal nouns the form is 'ag' before vowels and 'a' before consonants, 'ag òl' "drinking", 'a dol' "going" (and exceptionally 'ag radh' "saying"). However the 'a' [ə] is always unstressed and so is generally elided after a vowel in normal modern speech.
Manx is written rather as Scots G. is pronounced, so all that is seen of this 'ag' is a prefixed 'g-' before verbal nouns beginning with a vowel, and the exceptional 'ta mee gra' "I'm speaking" (= SG. 'tha mi ag radh'). Manx writes the preposition as 'ec'.
I'll leave it for others to comment in detail on how this works in the various Irish dialects, but AFAIK it is always written 'ag' although sometimes pronounced [ig].
Cornish and Breton both use their equivalent of Welsh 'wrth' "with, by, at". (cf. the W. idiom 'wrthi yn mynd' "in the act/process of going" etc., literally "at-it in going").
In Cornish the preposition is '(w)orth', which in the present participle construction becomes 'owth' before vowels and 'ow' before consonants, but with voiced stops being devoiced (and probably geminated) as a result of assimilation of the lost [-θ], e.g. 'owth + doz > ow(t) toz' "going".
In Breton the preposition is written 'ouzh' (no 'r') and the participle formed with 'oc'h' [ox] before vowels and 'o' before consonants. Unlike the devoicing (hard mutation) seen in Cornish, in Breton there is a 'mixed mutation' that is in origin a combination of lenition and devoicing ('soft + hard mutations') but naturally with its own pecularities!
As in Cornish the full prepositional form is restored when there is a pronoun object represented by a possessive pronoun before the verbal noun. So for example "seeing him" becomes literally "at his seeing" C. 'orth y welez' B. 'ouzh e welout', (cf. W. 'yn y weld', SG. 'ga fhaicinn' < 'aig a fh...').
These constructions, equivalent to English "he is/was/will be Xing" are very common in Welsh, Cornish, Scottish Gaelic and Manx, where there is no inflected future tense distinct from a generalised present*. Only Irish has retained this distinction, but Breton has restored it by re-purposing the inherited future subjunctive (still found in a few set phrases in Welsh e.g. 'cyn bo hir', 'da boch chi') as a simple future. This sort of makes sense as nothing in the future can really be known for certain
In Irish and Breton these construction seem to be less common (e.g. they're introduced about chapter 15 in beginners' courses rather than right near the start for the other languages) and rather place emphasis on the on-going action being described. At least that's my impression FWIW.
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* Edit : I should really have said 'non-past'. The exception is the verb 'to be' that is used as an auxiliary allowing distinctions more or less equivalent to English, "I struck, I strike/will-strike (habitually etc.), I was striking, I am striking, I shall be striking, etc. (SG. Bhuail mi, Buailidh mi, Bha mi a' bualadh, Tha mi a' bualadh, Bithidh mi a' bualadh).
Kyn nag ov den skentel pur ...